I don't think there is much of a difference. Perhaps *jetzt* is more often used in spoken language, while *nun* is more often written than spoken, although there may well be regional differences.

If you must make a difference: *jetzt* means "right now, this instant" where the meaning of *nun* is closer to "presently" or "by now". It's broader and can be used as a filler as well (in a fairy tale, say). For the most part *nun* and *jetzt* are synomyns, though.