The dominant mechanisms for motion of charge carriers in forward and reverse biased silicon p-n junctions are,

In normal PN junction the net diffusion process stopped due to the barrier potential developed across the junction, but in forward bias majority charge carriers get energy from the biasing potential and starts diffusing. Where as, in reverse bias the electric field supports the barrier potential so the only cause of movement of charges is due to the electric field and minority charge carriers, constitute the current due to drift.