Xatu can learn Giga Drain by TM in Diamond and Roost by breeding in White. Can Xatu learn both in White? I'd love to have it possible, just not sure if it'll work.
Xatu can get Giga Drain from the Pokemon Dream World or via Technical Machine (TM). It can get Roost as an Egg move as well from Pokemon like Aerodactyl, Archeops, Altaria, etc. or through a move tutor.