We characterize which 3-dimensional Seifert manifolds admit transitive partially hyperbolic diffeomorphisms. In particular, a circle bundle over a higher-genus surface admits a transitive partially hyperbolic diffeomorphism if and only if it admits an Anosov flow.Since the examples in [BoPP, BoGP, BoGHP] are obtained starting with an Anosov flow, they do not provide a negative answer to the previous question.