what’s the difference between ParallelGC and ParallelOldGC?

Take a look at the HotSpot VM Options:

-XX:+UseParallelGC = Use parallel garbage collection for scavenges. (Introduced in 1.4.1).

-XX:+UseParallelOldGC = Use parallel garbage collection for the full collections. Enabling this option automatically sets -XX:+UseParallelGC. (Introduced in 5.0 update 6.)

where Scavenges = Young generation GC.