Good question, the kickboxing I do has nothing to do with elbows. Obviously you use all kinds of kicks,knees etc in combination with punching. But what anyone who has not done both can't tell you is the movement is different as are how you hold your hands. In boxing you duck and otherwise hop around a lot, its different in kickboxing when you just duck a punch from a kickboxer your going to get the opportunity to smell his leg/knee/foot. And with the same movement you use in boxing your legs are entirely vulnerable to leg kicks. With your hands you tend to keep them higher because you have head kicks to worry about etc. Those are the big differences.
In my opinion boxing > kickboxing in a street fight against joe schmoo, but a kickboxer will typically beat a boxer in a fight. (for fieos, im talking 2 guys equal in every way, with the exception of fighting style