Historical Fiction; Hints of Surrealism and Magical Realism; a Dash of Autobiography
You can tell by that heading that we're having a problem committing ourselves on this subject. That's because Kundera uses bits and bobs from a variety of literary genres to explore his major themes.
We detect surrealism in Tamina's journey to and experience on the island, and in Kundera's intrusions into the narrative with stories about his dying father. There's also the inclusion of Thomas Mann's Death in Venice—not to mention those creepy, silent ostriches. Magical realism appears in the floating dancers and the angels of various varieties that populate the book.
All of this mix, however, is grounded in the inescapable reality that was Communist Czechoslovakia, and in the experiences that etched themselves into Kundera's memory. He's not interested in telling a unified story; what he wants is to write a novel "in the form of variations":
The various parts follow each other like the various stages of a voyage leading into the interior of a theme, the interior of a thought, the interior of a single, unique situation, the understanding of which recedes from my sight into the distance. (VI.8.1)
So we're not the only ones who lack complete understanding about Kundera's narrative form. We can say for certain that it is a novel about interior journeying, and about exploring issues that can't easily be articulated.