The Unbearable Lightness of Being is a genre-defying text if we ever saw one. On the one hand, it's like a philosophical treatise with extended examples to support its ideas. Much of the text breaks from the plot entirely while the narrator riffs on a variety of complex and interwoven philosophical concepts.
On the other hand, it is a love story for the ages – the relationship between Tomas and Tereza is epic stuff and at the emotional core of the novel. The importance of the historical setting is undeniable – see "In a Nutshell" for some background and "Setting" for some analysis – which qualifies the novel as historical fiction. You could argue that The Unbearable Lightness of Being is a modernist novel, because of its narrative trickery (see "Narrator Point of View"), non-linear presentation, and shifting levels of reality.