They are different in voice.
Passive voice: The house was burnt down (by Max).
Active voice: Max burnt down the house / Max burnt the house down.
Middle voice: The house burns down. <present tense>
Ergative: The house burnt down. <past tense>
___________________________________ In English middle voice is used to mean the construction where the patient is the subject of an intransitive but active verb. Compare:Active: John burns down the house.
Passive: The house is burnt down (by John).
Middle: The house burns down.
The agent is optional in the passive: The house is burnt down or The house is burnt down by John. The agent role is assigned to a prepositional phrase. But the middle has no agent slot at all. You can't say *The house burns down by John. Moreover, you can't use qualifiers that focus attention on the agent's role either:John burns down the house deliberately.
The house is burnt down deliberately.
(ungrammatical): *The house burns down deliberately.
John burns down the house for the insurance money.
The house is burnt down for the insurance money.
(ungrammatical): *The house burns down for the insurance money.
The English middle is therefore often used in situations where there is no agent, as opposed to an unknown agent, which may be indicated by the passive: Paint was smeared all over the walls (no agent expressed but we know someone did it), cf. This paint smears easily (not talking about any particular act of smearing). Read more here...