Can Kennen give Flow for a spell got discarded by Minefield when conquering with Kennen?

No. Kennen's target is locked when his conquer trigger is placed on the Chain, and at that moment Minefield hasn't resolved yet — so the cards it mills into your trash aren't there to choose.

Both "when you conquer" triggers go on the Chain at the same time (Rules 383.3–383.4), and targets for triggered abilities are chosen at finalization, not resolution (Rule 352.4.b). You can order Minefield first and Kennen second, but Kennen already locked his target before Minefield resolves — so the top 2 cards from Minefield are never eligible.

Side note: Minefield's mill isn't a Discard game action either, so it won't trigger "when you discard" effects. To use Kennen on a milled spell, you'd need that spell in your trash before the conquer happens — e.g., mill first, conquer later.