Half wave rectifier removes one of the positive or the negative half cycle of the wave and only either half of the cycle appears in the output whereas in the full wave rectifier both the cycles appear in the positive or negative cycle of the output.
Half wave rectifier: These type of rectifier converts only one half of AC into DC signal, either positive or negative cycle. As it rectifies AC partially its efficiency is also less.
Full wave rectifier: It converts entire AC into fluctuating DC that is it covert both half cycle to DC. Therefore its efficiency is almost double of half wave rectifier. Efficiency is 81.2 %.