Invasive cardiologists do basic left and right heart catheterizations for diagnostic purposes, but do not do interventions such as angioplasties/stents/rotoblades on blockages. So, when an invasive/noninterventional doc does a cath and finds a problem, he either calls one of his interventional partners into the cath lab to do an intervention, or the patient schedules another day in the cath lab with an interventional cardiologist.
When it is the invasive doc's turn to take night/weekend call, an interventional doc has to take backup call.
Intervention is one more year of fellowship, and of course, has higher compensation for the higher level of responsibility, although cardiologists of all types are paid well.