Both of them are different things.
The average kinetic energy is simply the mean kinetic energy a molecule possesses.
From equipartition theorem, this is given to be,
#E = (fk_BT)/2#
Where #f# is number of degrees of freedom of the molecule, #k_B# is the Boltzmann constant, and #T# is the absolute temperature.
On the other hand, if out of a total #N# molecules in a system, #n_1# Molecules have a velocity #v_1#, #n_2# molecules have a velocity #v_2# and so on, then the mean velocity or average velocity is defined to be,
#v_(mean) = (n_1v_1 + n_2v_2 + ....)/(n_1 + n_2 + ....)#
#implies v_(mean) = (sum n_iv_i)/N#
Where, #N = sum n_i# is the total number of molecules in the system.
