There is actually no difference between the three terms. In fact, the correct term is PVC-U.
The name changed from UPVC to PCVU in the late 1980’s, which coincided with our European neighbours. In European languages they mostly write the noun (PVC) first before the adjective (U).
PVC is an abbreviation of Poly Vinyl Chloride and U is an abbreviation of Un-plasticised. Poly Vinyl Chloride is used for many products, including bags, shoes and faux leather, which is why we are often familiar with the term PVC.
Interestingly, we cannot use pure PVC-U for windows and doors. Instead, a tiny amount of additives are added. This ensures the PVC-U can offer longevity and weather resistance, which is important when going up against the temperamental British weather! The additives also give the windows and doors a white colour.