Hillbilly - lives out in the sticks where they still don't have a lot of modern conveniences. Cowboy - typically from the American southwest. Wears cowboy boots, cowboy hat, tips his hat to ladies, and says "Howdy ma'am". Redneck - can live anywhere (but most often the south) and is characterized by a stern, conservative no-nonsense mentality. Often thought of as being racist but is not necessarily so. Typically rednecks are very patriotic.
Hillbilly=ghetto, very rugged appearance and they do weird things for entertainment such as shoot fish from lakes or creeks. Redneck=more of a farmer type of person. They live simple lives driving old trucks and killing and growing their own food. Cowboys=usually work on ranches cleaning stalls, hauling hay and of course herding cows.