Basically nothing! In 1992 the UK came into line with the EU for licencing. (HGV) Heavy goods vehicle became (LGV) Large goods vehicle. The vehicle categories also changed. This made the following changes.
HGV 1, Became LGV C+E
HGV 2, Became LGV C
HGV 3, Became LGV C1
PSV Became PCV which is now categories PCV D1, D1+E, D and D+E.
You will still hear both terms used, but it is the same thing.