Jenna ->
Jenny ->
Jennifer ->
Guinevere -> Gwenhwyfar
Jenny today is used as a pet form of
Jennifer, hence ultimately deriving from Gwenhwyfar.
Jenny used to be a nn of
Jane (a feminine form of
John, of course), and still is, I suppose. But today's Jennys are much more likely to derive from
Jennifer.
Though I'm still confused as to why
Jessica said
John meant "
Yahweh", forgetting the "is gracious" part. (No offence
Jessica!)
Miranda