If Kennen is hidden at a location then when he is revealed can he stun a unit at base or another battlefield

No — when revealed from Hidden, Kennen's play-ability stun must target a unit at the same battlefield where he was hidden.

Rule 811.1.d.2 restricts any target chosen by a play effect of a hidden permanent to that battlefield unless the card text makes same-battlefield targeting impossible. Kennen, Keeper of Balance's "Stun a unit" has no such escape clause, so it's locked to his hiding battlefield — base and other battlefields are off-limits.

One nuance: that's only true for the "When you play me" trigger. His attack trigger is a normal ability, not a play effect, so when he attacks later, the stun can hit any unit anywhere (including base or another battlefield).