Libvirt does not use KVM. Libvirt uses either QEMU (which in turn might or might not use KVM) or Xen or other hypervisors. So it's incorrect to say that Libvirt abstracts over KVM.
And virt-manager indeed manages Libvirt machines so it's not at the level of QEMU as you wrote in the parent comment:
> Proxmox is a virtual machine manager (like QEMU, virt-manager)
>Using KVM, one can run multiple virtual machines running unmodified Linux or Windows images. Each virtual machine has private virtualized hardware: a network card, disk, graphics adapter, etc.
Straight from their site. QEMU is the user space interface, KVM the kernel space driver. It’s enough to run whatever OS. That’s the point.
I don't want to necessarily make this an argument to/from authority, but for some context here - you are discussing this with Paolo Bonzini, maintainer of KVM, contributor to QEMU. In the list of people that best understand the difference and demarcation points between KVM and QEMU, he's pretty far up there.
And virt-manager indeed manages Libvirt machines so it's not at the level of QEMU as you wrote in the parent comment:
> Proxmox is a virtual machine manager (like QEMU, virt-manager)