That's the conceptual definition, but the actual process is slightly more complex.
For example, you could have Purchase indirect costs (landed freight, duty, etc.) in your cost sheet, and therefore incorporated into your standard costs. Or, you could be using Misc. charges of type Item to apply additional fees into your receipt inventory value (something I have not yet tried while on standard cost).
AX calculates PPV as the difference between the inventory receipt value it WOULD have applied to the inventory under FIFO and the inventory receipt value it MUST apply to the inventory under Standard cost. The key here is that more than just the purchase price can factor info the inventory value it WOULD have applied. Under the hood, AX does almost exactly the same work it would do under FIFO, and then just prior to posting makes a comparison to Standard cost when configured and offsets the difference appropriately. Thus, understanding FIFO in AX is still useful even here.