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.