Book of Exalted deeds gives the template of the risen martyr and the deathless creature:
Deathless is a new creature type, describing creatures that have
died but returned to a kind of spiritual life. They are similar in
many ways to both living creatures and undead. However,
while undead represent a mockery of life and a violation of the
natural order of life and death, the deathless merely stave off
the inevitability of death for a short time in order to accomplish
a righteous purpose. While undead draw their power
from the Negative Energy plane, the deathless are strongly tied
to the Positive Energy plane, the birthplace of all souls. In fact,
the deathless are little more than disincarnate souls, sometimes
wrapped in material flesh, often incorporeal and hardly
more substantial than a soul in its purest state.
Features
• 12-sided Hit Dice.
• Base attack bonus equal to 1/2 of total Hit Dice (as wizard).
• Good Will saves.
• Skill points equal to (4 + Int modifier, minimum 1) per
Hit Die, with quadruple skill points for the first Hit Die.
• No Constitution score.
• Darkvision out to 60 feet.
• Immunity to all mind-affecting effects (charms, compulsions,
phantasms, patterns, and morale effects).
• Immunity to poison, sleep effects, paralysis, stunning,
disease, and death effects.
• Not subject to critical hits, nonlethal damage, or
ability drain. Immune to damage to its physical
ability scores (Strength, Dexterity, and Constitution)
as well as fatigue and exhaustion effects.
Unlike undead, the deathless are subject to
energy drain. Like living creatures, deathless are
harmed by negative energy and healed by positive
energy.
• Immunity to any effect that requires a Fortitude
save, except for energy drain attacks, effects that
also work on objects, and harmless effects.
• Cannot use the run action.
• Uses Charisma modifier for Concentration
checks.
• Not at risk of death from massive damage, but
when reduced to 0 hp or less, it is immediately
destroyed.
• Not affected by raise dead and reincarnate spells or abilities.
Resurrection and true resurrection can affect the deathless if they
are willing. These spells turn deathless creatures back into the
living creatures they were before becoming deathless.
• Evil clerics can turn or destroy deathless creatures as good
clerics turn or destroy undead. Good clerics and paladins can
rebuke, command, or bolster deathless creatures as evil clerics
rebuke, command, or bolster undead.
• Deathless creatures gain the same benefits from
consecrate and hallow as undead do from desecrate
and unhallow, and they are hindered by desecrate
and unhallow as undead are by consecrate
and hallow. Hide from
undead and undeath to death
also work against deathless.
Detect undead and deathwatch also
reveal deathless, and allow the
caster to distinguish deathless
creatures from undead. Evil casters can be
stunned by overwhelming auras of deathless
creatures as good casters can be stunned
by overwhelming undead auras. Use the
“undead” line in the detect evil spell description
when deathless are in the area of a
detect good spell.
Deathless are healed by
disrupt undead and damaged by unholy
water as undead are by holy water.
Deathless are not affected by disrupting
weapons. Spells that have
greater than normal effect against
undead creatures—including chill
touch, magic stone, searing light,
sunbeam, sunburst, and wall of
fire—do not have these enhanced
effects against deathless
creatures. Deathless
take only 1d6 points of
damage per two caster
levels from searing light.
Spells such as command
undead, control
undead, create undead,
create greater undead,
and halt undead do not affect or create
deathless creatures.
• Proficient with its natural weapons and any weapons mentioned
in its entries.
• Proficient in whatever type of armor (light, medium, or
heavy) it is described as wearing, as well as all lighter types.
Deathless not listed as wearing armor are not proficient with
armor.
• Deathless do not breathe, eat, or sleep.
The Next Big one is the EIDOLON From ghostwalk
This class sub's levels and is perfect for a GM to transform a living party member or bring one back from the dead as a new class with a mission.
There is also the RISEN MARTYR from the Book of Exalted Deeds page 68:
This class is perfect for the GM to grant a single level in cause it's a prestige class and the first level is the hardest to achieve.
Best Answer
(dumb) 3.5 RAW:
That's the only particularity of a undead regarding healing (besides the fact it can't heal itself).
Let's say you target a zombie with Heal. The spell's description tells us:
So we have to read the Harm text rules:
It seems obvious we have to ignore the last line if we want to keep a little sense here. Basically what we learn is that casting heal makes us charge the zombie with negative energy that deals damage. The undead trait says undead can be healed with negative energy, not that every negative energy heals then, so it "makes sense" that you can harm it, with the heal spell, doing negative energy damage.
By the way, note that by casting harm on the zombie, you are supposed to make him gain HP through positive energy. That is the effect of the spell and does not trigger undead special ability.
Pathfinder rules on this are copy-pasted from 3.5, with the same nonsense.
RACP (rules as commonly played):
When you target a undead with positive energy, you make it take damage, when you target it with negative energy, you make it gain HP. That's pretty simple and the weird cases have to be houseruled by the GM.
I can't provide absolute evidence for the fact everyone plays with these rules, but this kind of material for example suggests undeads taking damage from positive energy.