I'm going to go out on a limb here and say the paladin has done nothing wrong to warrant losing his status, however I'm sure that his God would impose some sort of atonement on him (namely defending the town from the goblin incursion that is sure to occur now).
The paladin belongs to an order that believes all undead to be evil and destroys them, correct? Thus the paladin was justified in destroying Father Nayavovich since Fr. Nayavovich was a ghost and thus in the eyes of the paladin an abomination that should not exist. Should he have maybe gathered some more information before going into "Smite Mode"? Definatly, but he did not act against his code by destroying what turned out to be a "good" undead (in fact I will go so far as to say if he did NOT destroy the undead, he would lose his status for failing to uphold his god's dogma).
Was he wrong to kill the monks? Of course not.. they attacked him first. They were justified in attacking him, and thusly he was justified in killing them. If a Lawful Good person thinks you're an enemy and tries to kill you, you are still Lawful Good if you kill them in self defense.
In short: He does not lose his paladin status but if this is/was a PC then he should be "tasked" with protecting the village against some newfound menace as "pennance". If its an NPC then he should be tasked with guarding the village for the remainder of his life.