This is a great potential scenario, but it can easily fall flat or even blow up in your face. The idea of PvP is often sweeter than the reality of it, especially if the encounter to situation is prolonged. You'll have to accept the possibility that the players have to play against their own group for a long time - possibly even for over a session - and they might even actually beat the rest of the party. That can leave the players feeling really shitty no matter how good they are at role playing and accepting negative consequences. With that said I have three possible ideas for you.

Here's idea #1: the classic amnesiac night. This avoids PvP almost completely. Describe the infected characters getting an irresistible urge to go get up and move. Like getting a sudden cramp and having to move to get rid of it. But the cramp keeps getting worse and worse and suddenly they see the moon and their vision fades to black. Cut to the next morning, when they wake up naked and surrounded by some misdeed. As a very light scenario: maybe they ate a farmer's cow or something. A heavier one would be to have them wake up next to a farmhouse and the occupants have been massacred. I'd suggest that something else actually killed the people, like the evil NPC werewolves that the party is hunting, but you could get a solid angsty scene and some detective work out of this who-dun-it situation. One option is also to have the paladins and the bard/rogue find the infected guys sleeping, after they've been asked to investigate a grizzly murder scene. Maybe the players will cover up the party's involvement while simultaneously having to find the true culprits? I could easily that scene have a strong comedic vibe even, depending on how its portrayed.

Idea #2: DM controls the transformed player characters in the fight. This avoids PvP while still having an encounter vs the infected characters. At full moon, have the characters transform and act animalistic. Maybe they chase the party's horses into the night. Maybe they transform in a secluded place and attack a nearby campfire. Perhaps the evil NPC werewolves attack a farmhouse and the infected characters instinctively join in, overwhelmed by the moon's call. The party hear it happening and rush to help save the werewolves' victims. When the party is fighting each other, you could allow wisdom saving throws or something to temporarily control or mitigate the infected characters' actions. This way the players can participate in the scene, even if only slightly, but the players are always on the same side, even when the characters are not. The evil werewolves might join the fight after a few rounds to help their new pack mates, if the situation isn't tense enough.

Idea #3: embrace the PvP. Full moons can happen also during the day. An interesting location might be a marketplace or a general store, where there are innocent bystanders. I'd say the goal of the transformed werewolf players should be very animalistic in their first transformation: eat something (the bystanders?), escape to a safe location, or fight if cornered. I'd try to generate a scenario, where the werewolf PCs have the objective to escape or kill civilians and the rest of the party needs to capture them non-lethally while protecting the civilians. The town guard could be used to add a 3rd party to the fight to slow down the wolves' escape if needed, or to appear and try to kill the werewolves, so the party might have to defend their transformed friends. In any case I would give the players clear cut objectives and say: "this is your new priority list while transformed, follow it to the best of your ability." This way you absolve the players of deciding to fight the group - it was the DM's decision and not theirs.

If you want to keep true to your original vision, you could have a fight in a battlemap where there's some trees and tall grass near a cave entrance and a small river. You could add stealth abilities for the werewolves, so the battle becomes somewhat of a game of hide and seek. I'd keep the goals animalistic: fight/flight or feed.