Full Virtualization vs. Paravirtualization
/dev/sda is the first detected disk of IDE/SATA/SCSI type. In this case, emulated(full virtualized) by the hypervisor.
/dev/vda is the first detected paravirtualizated disk driver. It is faster than emulated sdX devices if both are referred to the same disk, because there are less overhead in its operation compared to an emulated drive.
Full virtualization vs. paravirtualization
Let's start with a quick discussion of two distinct types of
virtualization schemes: full virtualization and paravirtualization. In
full virtualization, the guest operating system runs on top of a
hypervisor that sits on the bare metal. The guest is unaware that it
is being virtualized and requires no changes to work in this
configuration. Conversely, in paravirtualization, the guest operating
system is not only aware that it is running on a hypervisor but
includes code to make guest-to-hypervisor transitions more efficient.
In the full virtualization scheme, the hypervisor must emulate device
hardware, which is emulating at the lowest level of the conversation
(for example, to a network driver). Although the emulation is clean at
this abstraction, it's also the most inefficient and highly
complicated. In the paravirtualization scheme, the guest and the
hypervisor can work cooperatively to make this emulation efficient.
The downside to the paravirtualization approach is that the operating
system is aware that it's being virtualized and requires modifications
What are paravirtual devices?
When running a virtual machine, the virtual environment has to present
devices to the guest OS – disks and network being the main two (plus
video, USB, timers, and others). Effectively, this is the hardware
that the VM guest sees.
Now, if the guest is to be kept entirely ignorant of the fact that it
is virtualised, this means that the host must emulate some kind of
real hardware. This is quite slow (particularly for network devices),
and is the major cause of reduced performance in virtual machines.
However, if you are willing to let the guest OS know that it's in a
virtual environment, it is possible to avoid the overheads of
emulating much of the real hardware, and use a far more direct path to
handle devices inside the VM. This approach is called
paravirtualisation. In this case, the guest OS needs a particular
driver installed which talks to the paravirtual device. Under Linux,
this interface has been standardised, and is referred to as the