Les Misérables is such a gigantic book that it actually covers several genres at once. For starters, it's an example of historical fiction because its second half is based on the June Rebellion of 1832.
On top of that, it's an example of realism because it delves (and we mean delves deep) into the unpleasant aspects of human life by focusing on the wretchedness and misery of the poor. But the key thing here is that we're not talking Naturalism, which would become all the rage in France a few decades later. But where naturalism is unrelentingly, crawl-into-a-hole bleak, realism leaves room for a little sentiment—like the sickly sweet love story between Cosette and Marius. Let's just say that if Hugo had been writing twenty years later, Cosette probably would have ended up dying of tuberculosis just like her mom.