It depends on the type of v12n. Paravirtualization and similar, the answer is sort-of while hard emulation is definitely yes. There are efficiencies in memory usage because the often will share the same kernel code and userland code, which are memory pages that can be deduplicated at the hypervisor level. Read more about type-1 v12n.
> All processes in a proper OS are already isolated and there is no need for VM.
No. This is not how things work in reality. (Ideally, yes because hypervisors are OS "duct tape" but there is no such readily-available OS with strict resource limits and hard enforced VFS and network isolation.) Isolation, sharing, and hard limits on RAM, CPU, networking, and storage (bandwidth, block devices, and IOPS) is beyond the capabilities of every major OS. This is why VMware and similar type-1 hypervisors exist.
> All processes in a proper OS are already isolated and there is no need for VM.
No. This is not how things work in reality. (Ideally, yes because hypervisors are OS "duct tape" but there is no such readily-available OS with strict resource limits and hard enforced VFS and network isolation.) Isolation, sharing, and hard limits on RAM, CPU, networking, and storage (bandwidth, block devices, and IOPS) is beyond the capabilities of every major OS. This is why VMware and similar type-1 hypervisors exist.