the photo diode is operated in reverse bias but the led is operated in forward bias ..

ans the photo diode are kept in rev bias becuz when light falls on the junction many e-p pairs are created near the junc these minority carriers cross the junction and contribute to the current generated  by the heat and so the current gets amplified

in LEDs the electrons(as current) moving in the forward biased diode cross the junction  the get combined with the holes and release a lot of energy  in the form of light

i hope u got it