It looks like Volvo is doing something a little abnormal where they have integrated the PCV valve with an oil trap and possibly a couple other components. Lets break it all down:
PCV Valve -- positive crankcase ventilation valve
This is used to pull gases that collect in the crankcase, through the intake to get burned in the combustion chamber. It is a valve to protect the engine from backfires as shown below:
This is how it hooks up on an engine:
Since there is suction these can get clogged with oil which Volvo looks to have addressed with their part.
A breather / breather box is a little different. it is merely a filter system that allows crankcase pressures to vent out the engine. below is a breather box used as an upgrade for VW bugs
and what they look like inside
they are typically accompanied with a catch can for the oil that finds its way into the box.
This is the simplest type of ventilation setup you can have also simply called a breather
it is just a simple filter sticking out of the valve cover.
There a couple Pros to using a PCV vs just venting:
- it will actively pull potentially damaging gasses out of the crank case replacing these with clean air. This is important because when your engine warms up many oil contaminants will evaporate and if left to precipitate back into your oil they can cause it to become acidic.
- the gasses will not just vent into the air we breath instead it will get burned and cleaned in the catalytics. This causes very little if any performance degradation.