In a p - n junction, the diffusion current is directed

In a $p$-$n$ junction, the concentration of holes is higher in the $p$-region and the concentration of electrons is higher in the $n$-region.

Due to this concentration gradient, holes diffuse from the $p$-side to the $n$-side, and electrons diffuse from the $n$-side to the $p$-side.

The conventional current due to the flow of holes is in the direction of their motion (from $p$ to $n$), and the current due to the flow of electrons is opposite to their motion (also from $p$ to $n$).

Thus, the diffusion current is always directed from the $p$-side towards the $n$-side, irrespective of the biasing.