It could depend on the context of the question. For example, if someone asks "How are you doing? I'm doing well. How about you?" Conversely, if someone asks "Do you have plans for the weekend? No, I don't. What about you?" In the last example, you could also respond with "How about you?"

"What is the difference?" is the correct way to write this question Both "How about you?" and "What about you?" mean the same thing. There isn't really any difference and they can be used interchangeably.