Virtual Machines And Devices / Drivers

I see a lot of questions on drivers and devices for Virtual machines. (Hyper-V, VMware, "XP mode", Virtual PC, etc)

Its important to understand that the devices the virtual machines see are not the actual hardware in the machine.

That is to say...the virtual machine might know that there is a network card available to use, but it doesn't know that the physical network card is a Broadcom 3c905 (for example). The virtual machine thinks that it is a pre-defined device that is written specifically for use in the virtual machine.

Same goes with other hardware, your Nvidia 9800 GTX would show up in the VM as a totally different piece of hardware. And this virtual piece of hardware does not have the features or power of the 9800 GTX nor does it require its device drivers.

Virtual machines use 2 types of devices. Emulated and Synthetic.

Emulated - Drivers/Devices that require a lot of physical box overhead to process for the virtual machine. Basically there is a lot of translation that the hypervisor (virtual pc, hyper-v, etc) must perform for the virtual device to utilize the physical device. These are the types of devices/drivers you will see used in Virtual PC and MS Virtual Server 2005 and some others.

Synthetic - Virtual Devices/Drivers that are highly optimized to perform very seamlessly with the underlying physical device. Hypervisor intervention/translation is kept minimal and therefore performance is greatly improved. Virtual machines using synthetic drivers are often said to be "enlightened". These are generally seen in bare metal hypervisors like Hyper-V and ESXi.

The important thing to take away from this post is that the actual physical devices in your computer are NOT what the virtual machine sees.

The network traffic from your virtual machine is passing through the physical network card, but the virtual machine will "see" a totally different device than what is actually in the computer. This goes for any device.