Shooting fireworks (with effects) don't count as coming from the player for damage purposes.

How to reproduce:

1. Get crossbow (I have mine enchanted with quick charge 3 and multishot 1 for speed but I think this works without them) and fireworks with effects
2. Shoot the crossbow at a mob (I used zombies that were already targeting villagers) so that the fireworks will damage the mob when they explode (putting the mobs by a wall and firing into the bottom of the wall makes this easier)
3. Continue until the mob is dead
4. See no XP drop

Note also that when I did step 2 with the zombies already targeting villagers, the zombies did not sense me attacking them even though I was in range.