Cr0 + 3 KNVO3 + 2 KOH 3 KNIIIO2 + K2CrVIO4 + H2O

This is an oxidation-reduction (redox) reaction:

3 NV + 6 e- 3 NIII (reduction)

Cr0 - 6 e- CrVI (oxidation)

Change the oxidation states