Preventive maintenance is a type of maintenance that is done to prevent faults before they occur randomly. It is simply done to replace unscheduled maintenance due to unexpected breakdowns. Unexpected behaviors in a production environment creates variability, and variability is an unwanted effect since it indicates that some things are running out of control at the shop floor, which will hurt due dates, plans, etc.
Preventive maintenance can be simply checking if everything is fine with the equipment, cleaning, or replacing a part before it fails. It creates a down time in the machine as an unexpected breakdown does. It means both preventive maintenance or unscheduled maintenance uses some capacity of the equipment. In terms of using capacity, preventive maintenance and unscheduled maintenance are not very different than normal production process except that they do not produce any throughput.
So the difference between maintenance and production is throughput in terms of capacity usage. The difference between preventive maintenance and unscheduled maintenance is controlability. If we assume that we have an hour of preventive maintenance and an hour of unscheduled maintenance, actually they both take the same time, so they use the same capacity on an equipment. But while we can control where we can allocate that one hour capacity use when we prefer a preventive maintenance, if we don't do that then that one hour can steal capacity of the equipment randomly any time as an unscheduled maintenance.
Then, while we can plan preventive maintenance with other production jobs, unscheduled maintenance is out of control. Or, is it? Let's leave it to discuss some other time.