What exactly is a “clean hit”

In the context of a 3D fighter, a fuzzy guard is basically a technique that allows you to block attacks in multiple hitboxes at once, usually low and mid (mid may also include throws). The big problem is that a fuzzy guard can effectively neutralize a good mix-up - that is, approaches or attacks that are intended to leave the defender guessing.

From what I've seen about the SCV fuzzy guard so far, it seems to hurt mix-ups pretty badly. The specific way it works has been outlined and demonstrated on YouTube:

By holding guard and tapping up (instead of holding up) your character will begin his jump animation but will not actually leave the ground. During that entire time your character is in a guard state and will block any moves that come in contact with him. However, if your opponent throws any low move (normal or special) the game will detect it and force your character to jump, successfully avoiding the low attack. This makes many mix-ups in the game easily avoidable, as long as you can get the tapping down.

IOW, tapping up while guarding will cause you to visibly start to jump, but instead resume guarding. At this point, if you get hit with a mid/high, you will block it. However, if your opponent throws a low attack while doing this, you will automatically jump and avoid it. The effect is that you are thus blocking multiple hitboxes simultaneously because the game made the "best choice" for you by either blocking or jumping.

YouTube demo (same video as in the post linked above; Note: contains some language):

Also note: In 2D fighters "fuzzy guard" often has a completely different meaning, but I won't go into that here since it's not immediately relevant.

This is quite a late response so it's possible you no longer play SCV (which makes me sad, because I love this game), but for the sake of anyone with similar concerns, I'll endeavor to provide an answer. Because of the general nature of your question, I will attempt to cover a large amount of information in a succinct manner, although this will come at the expense of comprehensiveness. Note that I will be using the standard notation, in which directional inputs are denoted by the corresponding numpad directions (assuming 1P side) and the buttons are ABKG, which are the horizontal, vertical, kick, and guard buttons, respectively. Now,

I had trouble opening up the opponent's defense[...]

There are general principles that hold true for all fighting games when it comes to getting around an opponent's defense. Generally, the only way to land a hit on an opponent is to force a situation on them through some action that necessitates a guess between two or more counteractions. This is referred to as a mixup. There are many ways to mix up an opponent, and the simplest are referred to as 50/50 mixups. In SCV, examples of these include (but are not limited to):

• Forcing the opponent to guess between a standing block against a mid attack or a crouching block against a low attack, where both attacks are not reactable Grabbing the opponent, which forces them to guess the appropriate grab break (either A or B)
• Forcing the opponent to guess between stepping to avoid verticals or blocking to prevent getting caught by horizontals
• Forcing the opponent to guess between attacking at disadvantage or subject themselves to another mixup

Now, none of these situations are "pure" 50/50s--there are a myriad of options to counter either action, and in many cases the optimal counter is specific to your own character. However, the general principles remain the same, so now I'll delve into Xiba's specificity.

If your opponent is statue blocking (apprehensive to do anything at disadvantage except hold guard), you can attempt to directly bypass their guard by using lows or throws. Throws are a complete necessity for all characters and should be regarded in the same scope as blocking--in fact, they're intrinsically linked, with throws being the de facto counter to guarding. A common misconception among newer players is that throws are "cheap" because they can be hard to counter. This is in contradiction with the competitive nature of fighting games, and it must be accepted that the easy option of blocking isn't and shouldn't be unbeatable. As a result, if your opponent is being overly defensive, throw the everloving heck out of them until they change up their tactics.

In addition to throws, Xiba has the privilege of having strong lows for use in mixing up, which many characters lack. kB immediately stands out from the rest of his lows (and indeed from many lows in the game) due to its properties. Most importantly, it has incredible frames for a move of its type. At only -7 on block it's completely safe; most lows are -16 or worse! This is balanced by virtue of the attack being vertical--despite being safe on block, the opponent can sidestep and punish you while you are recovering. Don't let this turn you off from kB though, because not only is kB safe on block, but it's +1 on hit, which is equally remarkable. Almost all lows that are both fast enough to be unreactable are disadvantage on hit (for example, generic 2Ks are all -2). What this means is that Xiba can create near-neutral (but slightly advantageous) situations by hitting the opponent with kB, so its benefits extend far beyond the damage of the move itself. Some examples:

• At -1, it's common for the opponent to attempt a fast, safe interrupt--the ur example being 2A. Despite 2A being a generally low-risk and effective option for interrupts, it has its weaknesses, and Xiba excels at exploiting them. He has a myriad of options, including (but not limited to): backdashing and punishing the whiffed 2A with 3BKK; using B+K to auto-GI the 2A, guaranteeing a free high-damage Rem B; using Critical Edge to auto-GI the 2A, causing CE to do 110 damage instead of the 60 damage it does without GIing anything; or using 44K or its Brave Edge (BE) counterpart to jump over the 2A and hit the opponent while they are recovering.
• Another common action at near-neutral frames is to utilize some form of movement to reduce the opponent's options, possibly causing them to whiff an attack and leave themselves vulnerable. Xiba's 6A is an extremely effective counter to movement of all types (quickstep and backdash are the biggest ones to worry about), and on a successful step counter it will knock down and combo freely into the BE version, and you should always attempt to transition into the BE on counter hits with 6B or 6A.
• Finally, the third most common response to this situation is to simply guard while at disadvantage. If you think your opponent will opt for this, simply repeat back to attempting to open up their guard.

So now that we've covered the basic tactics for bypassing the opponent's guard, let's talk about alternative methods. I'm referring to guard damage.

When a player blocks too many attacks, their health bar will begin to flash yellow (and later red), indicating that they are at risk of a guard burst. The amount of damage done to the opponent's guard varies for each attack, but in general almost no move will break the opponent's guard in less than 6 successive blocks. Some moves are incapable of causing a guard burst, but all moves that can be blocked do some amount of guard damage, and the minimum guard damage a move can do will result in the opponent's guard gauge reaching the critical point in up to 31 successive blocks.

The metagame involving guard damage is complex and very interesting, and there are many different styles of utilization. Some players prefer to focus almost entirely on directly bypassing the opponent's guard, usually as a result of their character's inherent weakness at inflicting guard damage. With Xiba, it's best to establish a middle ground, as he is competent at both bypassing and damaging guard.

The rule of thumb (to which there are exceptions) for guard damage is that the bigger, slower, and more avoidable a move is, the worse it is to block it. As a result, it becomes necessary to condition the opponent into a state of apprehension similar to if you intended to throw them. However instead of throwing them, you do a heavily guard-damaging attack and hope they block it. As you might have inferred, a general prerequisite for a move being a good guard damaging tool is it being safe when blocked. You don't want to be giving your opponent free punish opportunities.

Xiba's best moves for damaging guard are:

• 4B (breaks in 6 successive blocks; not too difficult to make the opponent guard)
• WR B (breaks in 9; fairly fast but when done as an instant While Rising input it's the same speed as 4B)
• 6K (breaks in 11.5; fairly slow but has evasive properties)
• 4B+K (breaks in 6 and is +1 on block; extremely slow but if you can get your opponent to block this it's massively detrimental for them)
• 44K (breaks in 9; only -3 on block)
• Rem B (breaks in 10; opponent might hesitate to step in reaction to the stance's activation due to Rem AK)

Now, when the opponent's health bar is blinking red, it's near a critical point and may be broken if they continue to guard. It is most commonly optimal to initiate a guard burst with different moves than you'd use to bring it to the bursting point initially--the reason for this is related to the amount of recovery big, slow moves have compared to faster "pokes". You can still burst with the above moves but your opponent will be desperately trying to avoid guarding, and in doing so will attempt to employ every other manner of defense he knows. As such you will probably encounter difficulty making him stand still long enough to block slower verticals, so it's usually easiest to break them with 6B, B, 3B, 4K, or similarly fast moves that can burst guard. (You can consult his wiki entry on 8WayRun or this guide to see which moves are capable of breaking and which aren't).

Finally some general advice on guard bursts: Plan where and when you're going to break them. If your opponent has 5 HP left and is ready to burst in the first round, it makes no sense to burst their guard and KO them instead of using a quick, effective round ender like 2K and saving the burst for later. As for the where you break them, if the round is young and the opponent has a lot of remaining health, try to set up a situation where a guard burst would give you a free wall combo or ring out. If you can't, always know how to maximize your post guard-burst damage. Good options include A+BBK BE, 3BKK, 44K BE -> 6BK -> A+BB, or bursting with 3(B) and transitioning into a Rem B combo. NEVER do a throw or CE after a guard burst--there's always better options for Xiba.

That about wraps up the basics of beating guard. Despite how technical it seems when put into words, always remember that it boils down to a direct battle of wits between you and the opponent. Now, let's move on to the next part: how to act when your opponent is pressing HIS advantage. Every character has different options to facilitate some form of offense. Opponents will generally attempt to enforce some specific tactic and will focus their gameplan around it. The nature of the character plays a large role in the effectiveness of different strategies, so you don't necessarily have to read your opponent's mind to find out what they have in store for you (for example, if someone tries to play Astaroth like he's Natsu, they won't be very successful).

[Additional caveat: very good players have strong fundamentals and excel at adapting their playstyle; these players are challenging to fight almost regardless of the character they're using and they often enjoy challenging themselves by seeing what they can get away with. If they can beat you using one move and one move only and they find it amusing to do so, they can and will attempt it unless you're good enough to adapt and force them to show respect. Don't get frustrated with this--everyone's a beginner at one point.]

Now, a brief elaboration on whiff punishing.

Whiff punishing in the literal sense refers to the action of attacking the opponent while they're recovering from a missed attack. When referred to as a strategy, it generally means the actions a player takes to cause the opponent to whiff. I touched upon the baiting of whiffs when I talked about kB. Situations like those crop up absolutely everywhere, which is why positioning and evasion are key aspects of SCV. Keep in mind that not all character movement was created equal--however, Xiba's is more than adequate. To cause whiffs, your most effective options by far are backdash and quickstep. Backdash, performed by tapping 4, will cause your character to take a step backwards. This is excellent for evading all forms of short-ranged attacks and can be used to get out of otherwise ambiguous situations. Against whiff punishing characters, the neutral game is heavily influenced by the 50/50 between using a long-ranged "backdash killer", which are usually verticals, or a short-ranged "sidestep killer", which are usually horizontals. Some characters have moves that are effective at countering both methods of movement, such as Aeon's 66A. Against these moves different tactics have to be enacted on a case-specific basis. Let's look at an example using Xiba versus Leixia.

Leixia's stepkill game consists mainly of fast, effective, short-ranged horizontals such as her AAB. She enforces these by using 66BB to counter backdashing. As a result, if she's trying to stop you from moving, you have a decision between sidestepping to avoid 66BB or backdashing to avoid her horizontals. If you guess right, she whiffs and you get a 3BKK whiff punish. Guess wrong and she gets a run counter. Effective keepaway and whiff baiting is just another mindgame between you and the opponent, and the best way to get better at it is experience and realizing what you did wrong when you get hit. Now, in your question you asked

Despite the flurry nature of Xiba's moves, however, none of the ones I tried seemed to simply start up and hit quickly to punish mistakes like this.

This is a much easier answer. 3BKK is Xiba's strongest whiff punishment tool. 3kB is a faster but less damaging alternative. Not all whiffs can be punished on reaction, but by making an educated guess and moving in a manner that would cause a whiff you can preemptively begin your attack to punish fast-recovering moves.

Now, your final questions:

What are Xiba's effective punishing strategies? When an opponent leaves himself open for just a few frames, which of his attacks are most likely to hit and interrupt the opponent's flow?

These are much easier to answer concisely and objectively. Xiba's not a very strong punisher relative to other characters, but he's not impotent. His options are:

• i13 - K
• i14 - 2A, 3A
• i16 - 3kB [This is his most important punisher]
• i19 - 3BKK For additional consultation, there is a guide to punishing with Xiba here

  What kinds of attacks does his "Rememberance" stance counter? Is using that even a reasonable option?

Remembrance counters all high, mid, low, special-mid, and special-low horizontals from frames 7 to 40. It also counters horizontal-like kick attacks, such as many 2Ks.

If any part of my answer doesn't quite make sense to you, there are additional resources I would like to link but am unable to. If you require clarification on topics like frames or notation, just tell me and I'll do my best to improve this answer.