For Nath and everyone else,
Last night I had a genius idea of how XL lives and has a really happy ending. I thought I will share my imagination with you, it's quite believable I think.
What happens is when XL saves XY life, he actually doesn't kill one of his nine lives but instead transfers it over his life to XY, it isn't dead, but living inside of XY... Sorta like this 
(Dont mind the 2min scribble) and then over time he slowly grows all his nines heads back and then departs from her body as XL, then a happily ever after