What is the difference between server virtualization and storage virtualization?

Server virtualization is the idea of abstracting the hardware away from the OS. It allows the creation of virtual "machines" that can then have an OS loaded on them. This in turn allows more than one OS to run on the same physical machine at the same time (as opposed to just having multiple OSes installed and booting one of them). The benefits include being able to better utilize your hardware and the ability to migrate the virtual machines to different hardware (which is also virtualized) for the purposes of server consolidation or physical server upgrades, as well as other reasons.

Storage virtualization is the idea of abstracting the physical hard drives or disks away from the OS. Basically presenting a logical representation of the disks to the OS. The benefits include being able to add or remove storage more easily, the ability to move data to other physical disks as needed without the server OS needing to know it, and the ability to pool storage so it can be managed independently from the server.

Both of these techniques have advantages in data center management, among them the ability to better plan physical hardware purchases or refreshes.

In Cloud computing, server virtualization allows the provisioning of standard servers on the fly (so no need to install another physical server). While storage virtualization allows pooled physical storage to be easily allocated where it is most needed.