aaronc..Hokie66 is correct. The subbase is important both from a support standpoint and a drainage consideration.
Also, keep in mind that pervious pavement ultimately becomes less pervious as the voids fill with debris. Some will tell you they are "self flushing". They're not. Minor flushing occurs at the surface, but below that no flushing occurs. If you are using it as part of your stormwater runoff system, consider that the runoff coefficient will be greater than gravel, but less than asphalt, at least initially.