Free Verse
If you had to label the poem as anything, you would have to call it "free verse," because it doesn't have a regular meter or rhyme scheme. The poem is almost like a speech or a dramatic monologue delivered by a character, the character being all of the Hollow Men. But the verses don't hold together like a speech in a play. In the last section, for example, a childish dance around the "Mulberry Bush" is contrasted with mysterious and philosophical lines about "the Shadow."
"The Hollow Men" consists of five sections of varying lengths. The lines are generally short. Like many of Eliot's poems, this one has an epigraph at the beginning – and not just one epigraph, but two.
Nor does Eliot use a strict meter like iambic pentameter, though he sometimes used regular meters in other poems. Nonetheless, with Eliot's deep knowledge of poetry traditions, we wouldn't be surprised if he were parodying some French or Italian poet – we just wouldn't be able to tell you who it was.