As I understand it (and I've been diagnosed AvPD by a psychologist who specializes in diagnosing personality disorders), the AvPD person has a MUCH deeper form of social anxiety than the social phobia person. It's easier to treat those with social phobia than it is an AvPD person.

The AvPD person is distrusting of others and feels inferior to them at all times. Any sign of another person wanting to be closer to an AvPD person will make the AvPD person shy away in fear, even though they may desperately want such attention. It's a very automatic response to avoid contact with other people and you don't even know you're doing it until it's done.

I suspect that the social phobia person may be a little more aware of how they are behaving even before they avoid the social situation.

For example, I will cross the street to avoid passing another person on the sidewalk if I'm out for a walk. I won't realize I've done it until I'm on the other side of the street.