What is best for covid-19 positive patient to take rest or exercise for enhancing immunity?

That depends: I wouldn't overdue it regardless. However, if one is asymptomatic or has mild symptoms but has NO fever, chest congestion, cough or GI upset, than mild to moderate exercise is probably OK. But if you have coughing, chest congestion, fever or gastrointestinal symptoms - than please rest. Exercising regularly is recommended if not infected.