I have polycythemia vera. Is it ok for me to wear extremely tight jeans or am I more likely going to get DVT or blood clots?

Probably OK. It is unlikely that jeans can ever create the degree of compression needed to affect circulation, and we in fact use compression to help treat patients with dvt.
Should be fine. Tight jeans is not going to add your risk for dvt- as long as you are active and not becoming sedentary. Follow up with your doctor and maintain your hemoglobin level with phlebotomy.
Agree that probably. Ok...Keep well hydrated & follow closely with your physician.