PVC resin is a hard material which is used for items such as pipe, gutters, window frames etc. With the addition of plasticisers, which make it more flexible, it can be used for tarpaulins, liners, flexible hose and other sheet type applications. Rather bizarrely, instead of calling PVC with Plasticisers added something like PVCP, the PVC without the plasticisers (i.e. that which is used for pipe etc) is given a "U" to denote that it is "unplasticised".
Technically then, the type of plastic used for rigid pipe is Unplasticised PolyVinyl Chloride (UPVC). This is the term that is widely (and correctly) used for window frames, doors, guttering etc. It is also sometimes used for industrial rigid pipe too, but industry has tended over the years, to drop the "U" so as to avoid confusion between industrial pipe and waste water / guttering pipe etc.
At the end of the day, if it is a rigid product, it doesn't matter if it is referred to as PVC, PVCU, UPVC, PVC-u etc. it's all the same stuff!