Do remember he's an NPC; as such, we should be talking CR, not ECL.
Now, we know Xykon is a sorcerer, meaning that he gets his first 9th at 18th level. The lich template adds 2 to the CR, making him a minimum of CR 20.
If he were a PC, then he'd be minimum ECL 22. Lich adds +4 ECL (sadly).