What type of poem is The Darkling Thrush? (i.e. sonnet, ballad,etc.)?

As for the 'style' of poetry, I am not sure, Hardy used several techniques throughout the poem to make the reader have a feeling of emphasis and presence. There is a frequent use of enjambment and the regular tetrameter, he also uses a regular ABAB rhyme scheme. If anything this poem gives a sense of rhythmical song.