[Ethereum] Extended private key vs private key

I am trying to make an hd wallet based on nodejs. So to have a secure wallet I have created wallet in my offline machine

const hdkey = require('ethereumjs-wallet/hdkey');
const bip39 = require('bip39');
var seed = bip39.mnemonicToSeed('my_mnemonic', 'my_password');
var chain = hdkey.fromMasterSeed(seed);
var xpub = chain.publicExtendedKey();
var xpriv = chain.privateExtendedKey();

Than using xpub I am generating addresses for users.

const wallet = require('ethereumjs-wallet');
const hdkey = require('ethereumjs-wallet/hdkey');
const _____ = require('dotenv').config();

const generate = async (index) => {

    const xpub = process.env.ETH_XPUB;

    const hdk = hdkey.fromExtendedKey(xpub);

    index = index || 0;

    const child = hdk.deriveChild(index);
    const w = wallet.fromExtendedPublicKey(child.publicExtendedKey());

    return w.getAddressString();
};
generate().then(v => {
    console.log(v);
}).catch(error => {
    console.log(error);
});

Ok, now I want to send coins from these addreses to my own address using extended private key – xpriv

const ethTx = require('ethereumjs-tx');
const Web3 = require('web3');


const web3 = new Web3(
    new Web3.providers.HttpProvider('http://localhost:8545')
);

//Verify connection is successful
web3.eth.net.isListening()
    .then(() => console.log('is connected'))
    .catch(e => console.log('Wow. Something went wrong'));


const params = {
    nonce: 0,
    to: '0x4584158529818ef77D1142bEeb0b6648BD8eDb2f',
    value: '0.1',
    gasPrice: 5000000000,
    gasLimit: 21000,
    chainId: 3
};
const privKey = new Buffer('xprv9s21ZrQH143K36eJpjZxNPYbNjzzUMvqLSG6P6Sex1unL8meREsNkcMNRw4ntv3WjJ2tJJX3CrdxkCJdyo7zLnjqdFppov2BAvY4iXYuMCY','base64');
const tx = new ethTx(params);

//Signing the transaction with the correct private key
tx.sign(privKey); //ERROR HERE


const serializedTx = tx.serialize();
Web3.eth.sendSignedTransaction(
    `0x${serializedTx.toString('hex')}`,
    (error, result) => {
        if (error) { console.log(`Error: ${error}`); }
        else { console.log(`Result: ${result}`); }
    }
);

When trying to sign I am getting an error that the private key is invalid. Waht private key I need to use to sign transactions.