${\text{HNO}}_2$ acts both as reductant and oxidant, while ${\text{HNO}}_3$ acts only as oxidant. It is due to their

In $\mathrm{H}\stackrel{*}{\mathrm{N}}{\mathrm{O}}_2$ oxidation number of $\mathrm{N}= +3$, N  can loose 2 electrons act as reducing agent. and can accept electrons so act as oxidising agent

In $\mathrm{H}\stackrel{*}{\mathrm{N}}{\mathrm{O}}_3$ oxidation number of $\mathrm{N}=+5.$ N can accept electrons and act as oxidising agent, N cannot loose electrons, as it is in its highest o.s