For relief from pain you can take 10 sittings of TENS (Transcutaneous Electrical Nerve Stimulation)at a physiotherapy centre. Give rest to the knee by wearing a knee support. Along with this you should do Static and Strengthening exercises of the knee.
Static: Sit in a long sitting position with back supported. Keep a small towel under the knee and try to press it down. Hold for a few seconds and relax. Repeat this 15/20 times twice a day.
Strengthening exercises: Sit on a chair with legs hanging down and try to lift lower leg and make it straight. Hold for few seconds and bring the leg down. Start with 4/5 repetitions and gradually increase no to 10/15. A small wt can be tied say 1/2 kg at ankle and repeat as above. Straight leg raising in lying position will also be helpful.