Can Dive Down protect a creature against Pacifism?

Question

If my opponent casts Pacifism on my creature and in response I cast Dive Down, what happens?

Answer 1

Dive Down resolves first (since it's the last spell on the stack) and will give your creature Hexproof.

Then, Pacifism (an Aura, which by rule 114.1b is a targeted spell) tries to resolve, but its target has become invalid, so it will not resolve and end up in your opponent's graveyard.

608.2b If the spell or ability specifies targets, it checks whether the targets are still legal. A target that’s no longer in the zone it was in when it was targeted is illegal. Other changes to the game state may cause a target to no longer be legal; for example, its characteristics may have changed or an effect may have changed the text of the spell. If the source of an ability has left the zone it was in, its last known information is used during this process. If all its targets, for every instance of the word “target,” are now illegal, the spell or ability doesn’t resolve. It’s removed from the stack and, if it’s a spell, put into its owner’s graveyard.

(source: Comprehensive Rules, emphasis mine)

Answer 2

Specifically, the comprehensive rules clarify that Auras (enchantments like Pacifism) require a target (emphasis mine)

303.4a An Aura spell requires a target, which is defined by its enchant ability.

608.3a If the object that’s resolving is an Aura spell, its resolution involves two steps. First, it checks whether the target specified by its enchant ability is still legal, as described in rule 608.2b.

Relevant portion of 608.2:

If all its targets, for every instance of the word “target,” are now illegal, the spell or ability doesn’t resolve. It’s removed from the stack and, if it’s a spell, put into its owner’s graveyard.

The rest of rule 608.2 only applies to instants, sorceries, and abilities. But auras do check their implicit targets on spell resolution. Then, as Glorfindel describes in their answer, Dive Down would make your creature an invalid target due to hexproof, so your opponent's Pacifism would not have any valid targets and be put into their graveyard instead of resolving.

Answer 3

Your creature would gain hexproof before the pacificim hit it therefore the pacifism would go to the graveyaard and your creature would be left on the battle field with hexproof and +0/+3 until end of turn.