A sloven is a disgusting, sloppy person. If you call your messy roommate a sloven, expect him to be offended.
You can use the noun sloven when you need a good word for a terrible slob. It's not a very common word, but it replaces pig very nicely when you're talking about a truly sloppy, unpleasant person. Sloven was first used in the fifteenth century to mean "person with horrible manners." Its origins are uncertain, but it probably stems from either the Flemish sloef, "dirty," or slof, which is Dutch for "careless or negligent."