Combine macaroni, butter, cheddar cheese, cream cheese, Parmesan, evaporated milk, whole milk, garlic powder, and paprika in a slow cooker. Season with salt and pepper.
Cook on high until the pasta is cooked through and the sauce has thickened, 2 to 3 hours, checking after 2 hours, then every 20 minutes after.
Garnish with chives before serving, if using.
