It used to be the case that, when selecting server hardware, a single operating system had to be decided upon. If one were to purchase a server with 8 CPUs and 16 gigabytes of RAM, all of those computing resources had to be devoted to just Windows or Linux applications, but never both simultaneously. However, server technology has evolved and this is no longer the case. Virtualization gives you the flexibility to mix and match operating systems as never before possible. Modern server processors now have multiple CPUs built inside, called "cores". Virtualization works by devoting a number of these individual cores to run a specific operating system. The choices include but are not limited to Microsoft Windows 98, ME, 2000, XP, Vista, 7, Server 2003, and Server 2008, as well as Linux distributions such as Red Hat, Debian, Ubuntu, Solaris, and Gentoo. More obscure operating systems are also supported, such as older Unix distributions, BeOS, and IBM's OS/2 Warp. This allows for the widest possible range of software to run on one server.
Not only is compatibility with both old and new software dramatically enhanced, but virtualization also makes your server utilization more economical. Rather than having multiple dedicated computer systems for each operating system you need to run, you can simply dedicate the individual CPU cores you need to each OS. Then, the other components, such as RAM, network adapters, display adapters, and storage devices are shared and allocated as you see fit between the different virtual machines. While virtual Windows Linux is one of the most popular implementations of this technology, the number of possible applications for it are virtually limitless.
Virtualization adds a new layer of security to your server implementation. Because each operating system is not aware of the other "virtual machines" running alongside it, software is not able to access memory and resources dedicated to the other operating systems. This minimizes visibility from the inside and as well as the outside of the server. Externally, if four operating systems are in use, the setup looks as if there are four different physical servers, each with their own IP address and network interface MAC address. These machines can also run with their own software and hardware firewalls, as well as their own security policy. A virtual Windows Linux approach maximizes flexibility while minimizing exploit potential.
That depends. Internal to the machine, the Windows and Linux operating systems are not "aware" of each other's existence on the same motherboard, blade, or server chassis. They appear to be separate computers on the network. Therefore, file sharing can be performed via the "Samba" protocol. This is a protocol commonly used for file sharing between multiple software platforms, including Windows, Linux, and Mac OS. This will allow you to establish shared drives on either virtual machine that are accessible from all other authorized physical and virtual machines, regardless of the operating system you choose to run on each.