No, we cannot find the mass of a photon by the definition p=mv . The equation p=mv is valid only for objects that move with a velocity that is much slower than the speed of light. The momentum of a relativistic particle like photon is given by pc=E2−m2c4​.A photon has zero rest mass. Therefore, on putting m=0 in the equation, we get p=cE​ , which is the valid equation for a photon.