Is Yellow Kennen cost when it attacks or is played can be reduced by purple ezreal ?

No — Purple (Chaos) Ezreal, Prodigy only reduces optional additional costs you pay (Accelerate, Repeat and similar), and Kennen, Keeper of Balance's stun is neither.

Kennen's "[2] to Stun a unit" sits in the "you may" slot as a cost within instructions, which makes it the base cost of the triggered ability that must be paid to finalize the trigger (Rule 383.3.b). Discounts on ability costs don't apply unless the effect says so (Rule 206.1), and Ezreal's text speaks only to optional additional costs on cards. The same reasoning is already confirmed in FAQ #12281 for Astral Heron's card-play discount — you still pay the full [2].

Same goes for Kennen's own 3 Energy cost, incidentally: that's a base cost, and Ezreal doesn't touch base costs either.