It depends on what you mean by curable. If you mean, change the personality entirely (such as making an avoidant not only capable of basic social interaction with minimal anxiety but very sociable and extrovert), definitely not. I do think any PD is curable if you mean minimizing it until it becomes more of the personality style rather than the personality disorder and the person can manage their life with minimal difficulty.
Current methods in psychology I think are mostly inadequate for dealing with PDs and difficult cases of other disorders like depression. Of course, they wouldn't be difficult cases otherwise. All disciplines always evolve and develop further ideas and schools of thought and psychology is a discipline that doesn't even have a very strong basis yet. All of this doesn't mean I think PDs are incurable, just that we'll probably have to wait sometime for more reliable cures, though there will always be some success in the meanwhile. Already, there have been people saying that DBT (dialectical behavioral therapy) is great for Borderline PD.
I don't even think it is impossible to cure Cluster B disorders. We'd certainly at least need more research to make it definite. You can't just declare something impossible before doing proper, thorough research and attempts at therapy. Even if this is found out eventually, such disorders should be removed from the DSM as they would no longer have any practical user in psychology, and you do have to be careful with the balance between helping people and judging "normalcy." I think it is often a bit too much to call anyone a monster either, they are human deep down somewhere, even if they refuse to act that way. Of course, if you do come across a potentially harmful person, the best thing to do is to keep yourself away from them and stand up to their behavior as necessary. It doesn't matter if they have a disorder or not, and ignoring them will feed their behavior far less than anything else you can do.
Anyways, enough mindless rambling. The American Psychological Association has a nice summary of an interesting article concerning some research that looked into the possibility of PDs changing, for anyone who is interested.
"Personality disorders may change over time":
Another, related article on developing PD treatment: