Xen solved a problem before 2007: no CPUs at that time had hardware virtualization extensions, and Xen allowed you to virtualize Linux-on-Linux without needing them.
However, the Xen developers never worked very hard on getting the code into the mainstream kernel, so literally for two or three years if you wanted Xen you had to run some ancient patched Linux kernel (2.6.18 IIRC).
From around 2007, CPUs started to be shipped that contained hardware virt. KVM was written about that time, and because it just defers virtualization to the hardware, the KVM code is small and simple, more akin to a device driver than anything else. Putting this in Linux was a no-brainer really.
Now, 2011, third generation hardware virt is everywhere, and it's really fast, much faster in fact than the original Xen "paravirtualization" trickery.
The Xen developers realized late in the game that they needed to get their changes upstream into Linux. The changes were late and very invasive, but now after ~3 years of trying, all the pieces are finally in upstream Linux, and that's the news you're reading about today.
However, the Xen developers never worked very hard on getting the code into the mainstream kernel, so literally for two or three years if you wanted Xen you had to run some ancient patched Linux kernel (2.6.18 IIRC).
From around 2007, CPUs started to be shipped that contained hardware virt. KVM was written about that time, and because it just defers virtualization to the hardware, the KVM code is small and simple, more akin to a device driver than anything else. Putting this in Linux was a no-brainer really.
Now, 2011, third generation hardware virt is everywhere, and it's really fast, much faster in fact than the original Xen "paravirtualization" trickery.
The Xen developers realized late in the game that they needed to get their changes upstream into Linux. The changes were late and very invasive, but now after ~3 years of trying, all the pieces are finally in upstream Linux, and that's the news you're reading about today.