I've taken a quote from here: Avoidant Personality Disorder (AvPD, APD)

"Along with the well-recognized overlap between avoidant personality disorder and generalized social phobia, evidence shows the distinctiveness of avoidant personality disorder. According to Millon (1996), avoidant personality disorder is essentially a problem of relating to persons, in contrast to social phobia, which is largely a problem of performing situations. Others have suggested that the one defining trait in avoidant personality disorder might be interpersonal sensitivity, whereas in social anxiety disorder, it is avoidance of social situations (Perugi et al. 1999)."

I really view it like that. Social anxiety is more situational, but AvPD is more intrusive(wrong word!). Like... with social anxiety, there are probably more situations that will cause the anxiety - like public speaking or asking someone for help in a store... anything specific (and there might be a lot of those situations for one person!). But AvPD is like... even when you're alone and no one's watching, you still feel like you're worthless and someone is going to find out and criticize you for it. It doesn't matter where you are, you don't have a safe-spot or situation.

That's sorta my take on it.