There is no difference. Those are just two words describing exactly the same thing but with different attitude towards it.
You ain't too likely to hear jealous girls saying: "Wow - she's sexy!" unless they're lesbians, most likely they gonna say "Wow - she's such a slut!" in a derogatory manner.
Some men, especially the insecure ones might have a problem with such dressing when they take their girlfriend out in the public, clubs, concerts or something like that, afraid that random guys might be hitting on her, whistling or yelling obscenities.
Any normal guy actually should be proud of it - other guys can act however they want but the girl belongs to me!
The proof of it is that no men - ever gonna complain that his girlfriend dresses "too slutty" when there's only they two around, secluded.
Sexy shows us an attractive image, a "silhouette" if you will.
Slutty shows us the path of least resistance.
Slutty means you're showing everything. From short skirts/dresses to bare midriffs. You're drawing attention to your body because we see it as the more skin you show, the more likely you're looking for "it".
Sexy just means, you know how to dress without showing off too much yet, not looking like a nun. Typically though, sexy is reserved for the bedroom. It's much better to be called "hot, pretty, cute and beautiful" when you're just out and about. It's more real
slutty = cheap and whorish. sexy = doesn't show too much/classy.
Slutty, to me, is trashy styling. Showing your thong, pulling tops down so your bra shows, tacky clothing, wearing bare threads of clothing, etc coupled with terribly applied makeup.
Sexy is wearing revealing clothing in a classy way. Using the "only show one" rule (show cleavage OR legs, but not both, etc). Deep cleavage looks better on women with small busts, because ladies with huge boobs are at risk of looking slutty if they show too much (unfair, but true). Ya know, all that.