My understanding is that it depends on the following vowel. ヴ is generally reserved for v/b sounds followed by "i" for instance (and ブ for "e"), but I have seen both イヴ and イブ, even エーヴ、イーヴ. There are also various kanji for "Eve" both phonetic and denotative, based on the meaning, if you'd be interested in those as well.