BEA Weblogic Server Virtual Edition doesn't run on Windows. It doesn't run on Linux, or MacOSX, or FreeBSD, AmigaDOS, CP/M, OS/2, or any other operating system you can think of.
It runs directly on an x86 hypervisor.
A hypervisor is a thin layer of low level code that sits just above the hardware and creates a virtualized version of that hardware. Usually several of them, so one server box can look like 2, or 10, or 100. Normally, an operating system like Linux or Windows or Solaris then runs on top of that layer, and your applications run on top of that. The technique has recently become popular because it lets administrators scale easily and unlock wasted potential in dedicated machines. With me so far?
Now, Java programs run on their own virtual machine, a pretend machine that runs "bytecode" instructions and has multiple threads and garbage collection. So now you have your code, on top of the Java VM, on top of the OS, on top of the hypervisor, on top of the hardware.
BEA said this is silly, and eliminated one of the layers - the operating system. As long as all your programs are in 100% Java, you don't need it. This frees up resources for more important things, like your application. BEA estimates they can reduce resource consumption by "25-50%" compared with a traditional software stack, though this sounds like a wild guess to me.
Of course, the things that an operating system does, such as process scheduling and memory management are still in there somewhere--they're just subsumed in, and customized for, the Java virtual environment. BEA calls the result "LiquidVM". Their white paper refers to "OS compression" which basically means they put in just the stuff you need and none of the stuff you don't.
The idea of a Java-only system isn't new. Azul Systems has been doing this for years on their specialized multi-way boxes. Even virtual machines and hypervisors aren't new; IBM pioneered the idea with VM/CMS on big-iron mainframes. But BEA is the first vendor to bring a compressed software stack and virtualization to cheap commodity Intel/AMD x86 hardware.
WebLogic Server Virtual Edition is now available for trial download from bea.com. Initially it only runs on the VMWare ESX hypervisor, but BEA plans to add support for Xen later this year and Microsoft Viridian after that.