Can be all three. If your tempdb has to grow to accommodate the data being loaded you are now using I/O. If the result set is large, especially if you are doing an order by, you will utilize memory heavily as the steps needed to prepare the results all will utilize memory ... if you don't have enough physical memory, then pagefile will be hit which now is doing additional I/O which also might make the CPU cycle up for the duration of the query ...
With enough resources, the CPU will spike, but the release is so instantaneous it is not an issue ... but what happens when the query is long running is that this high processor level may need to be maintained.
Hopefully that helps.
