I love
Sunniva. I also think
Sunny is a cute nickname but I think the full name is great in its own right. The sort of difficult pronunciation for English speakers is really the only reason I'd hesitate to use it.
I actually was babysitting once and saw that they had a
Christmas card on their fridge from a family with daughters named
Sunnifa and
Violet, and I thought that was interesting, both because I'd never heard of anyone with any variation of this name before, and that there was such a popularity discrepancy between the two names.
http://www.behindthename.com/pnl/87410