AvPD is an axis II disorder. SA is an axis I. But they do have lots of overlap in symptoms. Most people who have AvPD (like me) develop SA or GAD. I think I'm more in the GAD category. But a person could have SA without having AvPD. Here's the way I have heard the difference:
In Personality Disorders in Modern Life, they explain very well the difference between Axis I disorders and Axis II disorders. With that information people should distinguish easily an Axis I disorder like Social Anxiety and an Axis II disorder like Avoidant Personality Disorder.
They explain that:
Axis I disorders are analogous to diseases. They are ultimately the reason why you end up in psychiatric treatment. Those are depression, anxiety, depersonalisation, psychosis, and all those mental disorders that you can have. So they are analogous to the flu, the cold, the plague, etc: this is why you end up in the hospital.
Axis II disorders are analogous to the immune system. Personality disorders are analogous to a compromised immune system. In normal people, events in life are unlikely to lead to an Axis I disorder. However, if you have a personality disorder, you are almost certain to develop Axis I disorders, since it's as if you have a weakened immune system: any minimal infection leads to development of a disease. And like with compromised immune systems, you can also have something analogous to auto-immune diseases where the mere fact of having a personality disorder will lead to an Axis I disorder without requiring any life event at all. ; similar to auto-immune diseases like Lupus and Multiple Sclerosis, where the disease is caused by the immune system itself. The "immune system" is so overreactive that it ends up destroying instead of protecting the person.
For example, if you see Avoidant Personality Disorder as a compromised immune system (a defense mechanism, because that's really what it is), then you can see what "diseases" it makes you particularly prone to. A defense strategy that involves avoiding difficulties will simply reinforce the tendency to "avoid", such as increasing the fight or flight response, making you more and more prone to anxiety over time. It will become very easy to develop a "disease" like social anxiety even under minimal pressure, and with strong cases of avoidance, the "immune system" itself can end up spawning social anxiety by itself without even needing an outside agent. Same thing that this immune system makes you very likely to develop depression over minimal life event, and even develop depression "just like that" with no life event attached to it.
So Social Anxiety Disorder and AvPD are not even the same "category" of disorders. Axis I and Axis II are not based on the same assumptions. "Anybody" is at risk of developping SAD when under suffisant pressure, but somebody with AvPD will need significantly less pressure, and AvPD can go as far as to spawn SAD by its mere presence.
In other words, if you have an Axis II disorder, Axis I disorders will be the story of your life. You can get immediate help for Axis I disorders, but Axis II disorders tend to be particularly hard to solve and many people simply take the easy route of solving the Axis I disorders that the Axis II spawns.