It is hard to draw a distinct line between the two. In many AvPD groups I'm in on Facebook there have even been arguments about whether the two can be comorbid. Some believe that AvPD is a severe form of SAD, meaning they believe they cannot be comorbid. Others believe that AvPD is something that occurs after a lifetime of SAD, thus can be comorbid. I tend to agree with the latter.

I think of AvPD as the result of years of social anxiety. Most people with social anxiety believe they can get better. However, with AvPD (and most other personality disorders), the person believes that the trait has become an integral part of them. To be honest, recovery scares me because of this. I can't imagine not being avoidant, because I think of it as part of my personality by this point. But someone with social anxiety would likely not feel this way.

Avoidants tend to... well, avoid. Just like people with SAD, they long for social connection, friends, etc. However, we tend to feel inadequate. So, unlike people with SAD, we often don't even try to connect. Someone with SAD may or may not try. In short... a person who is avoidant can have symptoms of SAD, and may be diagnosable with both. But not everyone with SAD will be diagnosable with AvPD.