Xenos is the name given to every nonhuman species in the galaxy. The Imperium of Man has a special hatred for Xenos due to the fact that most alien species encountered by humans have either betrayed, manipulated, or attempted to destroy humanity in the past. In fact, consorting with aliens is considered a capital offense and outright Heresy. Individual Imperial armies may choose to form temporary alliances with Xenos if only to deal with a more urgent threat, though backstabbing from either side is not uncommon.
While it is possible for aliens to be corrupted into Chaos worship, they generally do not appear in lore as most of the more noteworthy species are immune to corruption (Orks, Tyranids, Necrons), while most Chaos Space Marines maintain their hatred of xenos from their days as loyalists.