I'm really liking the name Fiva for a daughter.
Let me say real quick, yes this is a legitimate name. It's an old Russian name, origin a little confusing, but it either comes from the same Greek root as Phoebe, or the Greek name Thebe.
Anyway, it's super rare in English, so I wouldn't say there is a 'correct' spelling of it. My favorite spelling is Fiva, but I'm afraid people would pronounce it Fih-va, instead of Fee-va, like it's supposed to be. But Feeva looks funny written out to me.
So.. Fiva? Feeva? Other spellings?
Update: I would also be okay with a name that is similar, as long as Fiva would be an nice nickname to go with it.