I've run Xnest server. It appears Xnest doesn't use my graphic card or at least it doesn't use direct rendering.
Normal Xorg glxinfo
OpenGL vendor string: Tungsten Graphics, Inc
OpenGL renderer string: Mesa DRI Intel(R) Sandybridge Mobile
OpenGL version string: 3.0 Mesa 8.0.3
Xnest Xorg glxinfo
OpenGL vendor string: VMware, Inc.
OpenGL renderer string: Gallium 0.4 on llvmpipe (LLVM 0x301)
OpenGL version string: 2.1 Mesa 8.0.3