Some applications just weren't meant to be virtualized. They require too much CPU bandwidth or generate too much graphics overhead to run reliably in a VM environment, forcing their users to juggle multiple, physical PCs in order to run more than one application at a time.
A good example might be a computer-aided engineering package. Designed with the assumption that they'll be the only application running on a given system, these programs typically require powerful, workstation-class hardware. And while it may be possible to have these applications take turns hogging that hardware, the inevitable conflicts involving OS version, service pack level, and device driver support often make such co-habitation arrangements untenable.
[ Compare VMware Workstation 7, Parallels Desktop 4 for Windows and Linux, and Sun VirtualBox 3.1 in "InfoWorld review: Desktop virtualization for Windows and Linux heats up." ]
Enter Parallels Workstation 4.0 Extreme, a desktop virtualization solution that makes it possible to install and manage dedicated guest OS and application environments within virtual machines that provide near-native performance for graphics, disk, and network I/O. Parallels Workstation Extreme accomplishes this by isolating and assigning physical hardware on a per-VM basis, then allowing each VM to directly control the hardware to which it has been assigned.
For example, you might install Windows XP and Windows Vista into separate VMs and give each VM direct control over a single video adapter and NIC. The OS, device drivers, and applications within the VMs then run at nearly full speed against these dedicated resources, while the thin layers of isolation erected by Parallels Extreme serve to keep the VMs aligned with their respective dedicated resources.
Of course, the above scenario assumes a system with multiple NICs and video paths. Furthermore, the system's CPU and chip set must support Intel's Virtualization Technology for Direct I/O (VT-d), which is still uncommon outside of high-end workstations and servers. Clearly, this is a specialized solution, one that needs to be carefully matched with a compatible hardware platform. So it comes as no surprise that Parallels Extreme supports only select Intel Xeon workstations from the likes of Dell, HP, Lenovo, and Fujitsu.
Operating system support is similarly exclusive. Parallels Extreme requires a 64-bit Windows (XP SP2, Vista SP1, or Windows 7) or 64-bit Red Hat Enterprise Linux 5.3 host, and supports 64-bit Windows, 32-bit Windows, and 64-bit Red Hat Enterprise Linux and Fedora 10 guests.