I pressed the donk with KK and he pressed back.... what to do??

[Baintz]

Don't like turn raise, just call the small bet. Yes, it often means you'll face a decent bet on the river from someone so aggressive, but I would probably call that too (dependent on size and the river card). Sure, you'll lose sometimes, but you'll also pick off some bluffs.
Look at it from his point of view, your turn call says your hand is weak, so would he bet large on the river with a good hand into an obvious weak one? From an aggro player, I'd expect a larger bet to be more likely to be a bluff. I he bets small, you have an easy call and got to showdown cheaply.

As played once he 3-bets the turn, it's very unlikely he has a draw, as your line is super strong so he can't expect to have much (if any) fold equity.