I've always wondered how paravirtualizing some functions such as I/O or networking affects security.
Say a VM gets compromised, and is able to do what it wants with the block devices, how tough would it be to get out of the VM? If malicious code is able to access the host's block device that runs in kernel mode and start running code directly on the host's OS, game over.
Unlike Hyper-V and Xen, in KVM a paravirtual device looks an awful lot like an emulated device. For instance, virtio-net appears to the guest as a normal PCI device. It's quite conceivable that a hardware vendor could implement a physical virtio-net card if they were so inclined. In our backend, we implement virtio-net like any other emulated device.
This means from a security perspective, it's just as secure as an emulated driver. It's implemented in userspace and can be sandboxed as an unprivileged user or through SELinux.
VMware uses a similar model. Hyper-V and Xen prefer to not model hardware at all and use special hypervisor-specific paths. From a security perspective, the fact that these devices are on a different code path means that they have different security characteristics than emulated devices. For instance, in Xen, a paravirtual network device is backed directly in the domain-0 kernel so an exploit in the xenpv network device is much more severe than an exploit in a Xen emulated network device (since the device emulation happens in an unprivileged stub domain).