What are the differences between Astrobiology, Exobiology, and Xenobiology?
Each deals with different subsets of the study of life. See below for definitions.
Biology is the study of living organisms. The three specific types of biology listed in the question are:
Astrobiology is concerned with the study of life on Earth and in Space.
Exobiology is concerned with the study of the likelihood of the existence of life on other planets and what form(s) it might take.
Xenobiology is concerned with forms of life that aren't known to science and nature as yet. It's a subset of synthetic biology and deals with the creating, synthesizing, and manipulating of life systems.