SA is when social situations cause you anxiety, AvPD is when you avoid them because of the anxiety. Many people suggest they are the same thing, or AvPD is a severe form of SA. However, personality disorders are much harder to treat than anxiety disorders, so either they're inappropriately named, or they're different things.
The only difference for me is I can be diagnosed with SA (have been, in fact), but I can't be diagnosed with any personality disorders 'til I'm 18.
I do think I have AvPD though. It doesn't just suddenly appear on your 18th birthday.