3 KClO3 (aq) + 3 H2SO4 (aq) → HClO4 (l) + 2 ClO2 (g) + 3 KHSO4 (aq) + H2O (l)
This is an oxidation-reduction (redox) reaction:
ClV - 2 e- → ClVII (oxidation)
2 ClV + 2 e- → 2 ClIV (reduction)
KClO3 is both an oxidizing and a reducing agent (disproportionation (dismutation)).
This is a gas evolution reaction, ClO2 is the formed gas.