Q:

Does kha'zix passive trigger on ult while he is still visible?

If you set a pink ward and kha'zix ults within the reveal area, will the unseen threat passive work anyway?

A:

If your passive is already up you won't lose it, however if you have already consumed the buff any method of revealing will prevent getting the passive again. This counts for  warded bushes, Warwicks Blood Scent and diverse traps. The only exception is your Ultimate. You can be revealed during your stealth but you will still get the passive. Source (The site was deleted in english but is still available with spanish ability titles somehow)
On the other hand every kind of "stealth" will give you the passive buff. Most common used are: fog of war and bushes. You can however also get the passive by Nocturnes Paranoia, your ultimate and even Graves Smokescreen.

