If enemy cull the weak can I react to reveal a hidden Kennen at a battlefield and kill that instead of a different unit

Yes — that works.

Cull the Weak doesn't target ("Each player kills one of their units"), so you choose which unit to kill at resolution, not when the spell is played (Rule 355.10.e). Your hidden Kennen, Keeper of Balance has Hidden, which grants it Reaction while facedown (Rule 811.1.b), so you can play it from facedown in response to Cull the Weak — it must enter the battlefield where it was hidden, which is fine (Rule 811.1.d.1).

Kennen resolves first (LIFO) and enters play; when Cull the Weak then resolves, you're free to choose Kennen as the unit you kill — or any other unit you control. If Kennen is the only unit you have when Cull the Weak resolves, you must kill it.