The standard pronunciation is for this stem to be realised with/f/ when the preceding syllable has primary stress (as e.g. inNerven,entnerven,nervig), but with/v/ when the stress moves forward (as e.g. innervös,enervieren). However, a minority of speakers may use/v/ in all prevocalic positions. Compare the same inLarve.
Nerv on the German Wikipedia.Wikipediade