In pn diode, how does an electron flow from p to n during reverse recovery phase
When a PN junction is forward biased there will be excess minority electrons in the P Region due to diffusion of majority electrons in the N region into the P Region.Conversely, there well be minority holes in the N region.

When you reverse bias the PN junction the force(forward voltage) that allows for Electrons and holes to move to the P and N regions,respectively,is gone.

The question is where do these excess electrons and holes go?

Because of the electric field in the depletion region(from the N side to the P Side),excess electrons in the P side will be swept back to their original position in the N side(Electrons move in the opposite direction of the E-field).In the same Manner,holes(moving with the direction of the E-field)in the N side will be swept back To the P region.

This process causes the large reverse recovery current the moment a PN junction is Reverse biased.

As holes and electrons move back to the P and N regions,respectively,the depletion Region starts to respond the reverse voltage and widens to prevent further migration of holes and electrons and only small reverse current(leakage current) will pass Through the diode.