ProxmoxVE uses a lightweight perl interface to KVM, with a smaller footprint than the libvirt library, that's a C variant AFAIK.
It's a minimal bare-metal server-installation.
QEMU's pretty quick, especially if you remove the vCPU compatibility & let the guest have at the host CPU functions directly.
Recent improvements to paravirtualized drivers have allowed network throughput to jump approximately 8x what they were with previous offload methods.
QEMU also lets guests have a physical disk from the host as their virtual disk, removing the overhead involved in using disk images, although .raw images on LVM is still quite fast & provides the valuable snapshot functionality.