I was diagnosed with synovitis too, during an arthroscopy I had in September 2004. I could not heal from it either. All synovitis means is inflammation of the joint lining. I think the goal now is to find out what caused the synovitis. (It is a first step diagnosis when they can't tell you what else it is.)
As far as the PVNS, your orthopedist could have tested the lining of the synovium if he took a biopsy during surgery. Mine did and I tested negative for PVNS. I don't know much more about PVNS except it can recur--not sure if it is an autoimmune disease or not.
If you have not been to a rheumatologist, I would certainly recommend it. They can do a lot of tests to try to find the cause of the synovitis. Keep in mind it can take months or years for blood work to show up any meaningful information, however. It is a slow and inexact science that can be frustrating, esp. to Type A's like myself. The real urgency is to get the swelling under control so you don't do damage to your knee joint.
I imagine there are orthopedists who specialize in arthritis, although probably OA vs autoimmune? That would be another resource to talk to.
Apparently more than 1cc of fluid on the knee can cause damage, so it's important to get that under control. I have had my knees drained also and in addition, cortisone shots, but you have to be careful not to do either too many times to not cause further damage.