XPS is actually a type of extruded polystyrene (it actually stands for extruded polystyrene insulation), so there isn’t a difference between the two.

It’s a specific method of producing polystyrene insulation where the material is melted, mixed with additives and then extruded into the shape of the board. This process results in a closed-cell structure that offers excellent insulation properties, moisture resistance, and durability.