VirtIO was originally developed as a device para-virtualization as part of KVM but it is now an OASIS standard: https://docs.oasis-open.org/virtio/virtio/v1.3/virtio-v1.3.html which a number of hypervisors/VMM's support.

The line between what a hypervisor (like KVM) does and what is delegated to a Virtual Machine Monitor - VMM (like QEMU) is fairly blurry. There is always an additional cost to leaving the hypervisor to the VMM so it tends to be for configuration and lifetime management. However VirtIO is fairly well designed so the bulk of VirtIO data transactions can be processed by a dedicated thread which just gets nudged by the kernel when it needs to do stuff leaving the VM cores to just continue running.

I should add HVF tends to delegate most things to the VMM rather than deal with things in the hypervisor. It makes for a simpler hypervisor interface although not quite as performance tuned as KVM can be for big servers.