Renewal = It has expired, you do something to get it back.
Test required, has to be in an actual aeroplane.
Revalidation = Rating is still valid, you do something to keep it before it expires.
Can be done in a variety of ways, but if done by test, every OTHER test has to be in an actual aeroplane - you cannot revalidate by test in an FNTP II twice in succession.
In reality (assuming MEP) you fly an airways sector, IFR airwork and a couple of approaches (one asymmetric) as part of the MEP revalidation / renewal test, not a separate test, so the hassle and extra cost is not huge in the grand scheme of things.