what can cause white smoke out the tailpipe? With no water in the engine oil?

White smoke is generally caused by water vapors and could be caused by a myriad of things without showing in the engine oil. Same thing applies if the smoke is more of a bluish tint which would be oil vapors. I recommend you take the car to have it diagnosed by a Volvo specialist with the proper knowledge.

Water in the fuel can cause this as well. Try putting some dry gas in the tank as their could be a high water content in the fuel in the tank.

black smoke

Lots and lots of white smoke is indicative of a head gasket leak. If there is no water, white foam, in the oil, it just means that the leak is away from the oil passages.