How've you been?

How've you been?

How are you? "How've" is an informal contraction of "how have." Hey, great to see you again! How've you been? Hi Murray, how have you been?

How (have) you been?

a standard greeting inquiry. (See also How you is?) Bob: Hi, Fred! How have you been? Fred: Great! What about you? Bob: Fine. Bob: How you been? Sue: Okay, I guess. You okay? Bob: Yup.
See also: how