Xtina is Christina's "alter ego." She portrays her when expressing her sexualized image. She debut the persona during her Stripped era. It's also a play on of her name X = Christ (such as Xmas)
Xtina = Chris + Tina = Christina
I read it off somewhere in an old interview that her friend saw the relationship between Christmas and X'mas, since "christ" is X'ed, so she became Xtina