I attended devconf.cz 2014 and gave a talk on VIRTIO 1.0, the standard for paravirtualized I/O devices supported by KVM and other virtualization software.
If you are looking for an overview of virtio, curious how paravirtualized I/O works, or thinking about designing custom devices, check out the slides:http://vmsplice.net/~stefan/virtio-devconf-2014.pdf
The VIRTIO 1.0 standard refines the hardware specification so that implementors have a clear reference. Older versions of the specification were informal and did not undergo as much scrutiny. The presentation explains the key concepts: virtqueues, configuration space, the device status field, and feature negotiation. It should give you enough background to dive into the code or standard.