In forward bias, depletion layer decreases and the diffusion takes place which results the electric current from p-side to n-side. So, the diffusion current increases while drift current remains constant in forward bias. In reverse bias, the width of depletion layer increases, so the diffusion becomes more difficult, so net current (in very small magnitude) is due to the drift. Therefore, dominant mechanisms for motion of charge carriers, diffusion in forward bias and drift in reverse bias.