There is no difference between them. But the verb "kiffer" is slang (familiar), it's just used in spoken french to say that you love someone or something, and it's mostly used by teenagers.
Actually it's derived from the arabic word "Kif", which means, roughly translated, "sense of well-being" ( “Kiffer” is a derivative of “kif”, which comes from Arabic meaning “hashish” (or at least something related to hemp, I’m a bit short on serious references), figuratively “sensation of pleasure (due to hasish)” )