4 Fe0 + KNVO3 + 10 HCl N-IIIH4Cl + 4 FeIICl2 + KCl + 3 H2O

This is an oxidation-reduction (redox) reaction:

NV + 8 e- N-III (reduction)

4 Fe0 - 8 e- 4 FeII (oxidation)

Change the oxidation states