To be xenial is to be hospitable. Now that may seem contradictory considering Joe's reclusiveness, but in actuality, he's very xenial with Michael.
He comes out to him when Michael is still a complete stranger to him. Joe makes himself completely vulnerable to someone he doesn't know or trust. But he comes out as a way to extend his hand out to Michael when Michael says that he is gay. He's clearly nervous, and he just explained to Joe how he moved to Detroit from Phoenix to be with a man who was married to a woman. So he's scared and lonely. Joe admits he's bi so that Michael doesn't feel judged. He uses it as a way to reach out to Michael, to make him feel more comfortable and accepted.
Further still, Joe opens his home and his life to Michael. He spends quite a bit of time with him, which Joe doesn't normally do. Whether they go out, or just relax in Joe's apartment, the two men spend quite a bit of time together. For Joe to allow someone other than Claudette into his home, or to spend his free time with someone other than Pete or Claudette is as xenial as he can get. It may not rival southern hospitality, but it is quite hospitable nonetheless.
Keeping things relative, you can see that Joe is actually very hospitable with Michael. He opens himself and his home as much as he can. Joe, by nature, is a loner, but he is a xenial loner.
