The back-ends available in KDE SC 4.7 for this compositing window manager are: OpenGL 1.x/GLX, OpenGL 2.x/GLX, OpenGL ES 2.0/EGL, X Render, and No Compositing (not a back-end but just KWin doesn't do compositing).
The OpenGL 1.x back-end was what was used by KWin up through the KDE SC 4.6 series. With KDE SC 4.7 is where the OpenGL 2.x support was introduced along with OpenGL ES 2.0 for the mobile platforms. With KDE SC 4.7 and going forward, the GL1 back-end is considered the legacy back-end and as implied by it's name, just uses fixed-function OpenGL 1.x calls. This legacy back-end is only used if the graphics processor / driver doesn't support or advertise GL2 or if the GL2 support for KWin is explicitly disabled from the advanced desktop effects settings. Interestingly, the AMD Catalyst (fglrx) driver is blacklisted from using the GL2 back-end, so for now it also is using the GL1 back-end even though the Catalyst driver on modern hardware supports GL3/GL4.
The new GL2 back-end with GLX is the default compositing back-end the rest of the time. This new back-end to KDE SC 4.7 uses a programmable pipeline and rendering is done using GLSL shaders. GLX with the texture_from_pixmap extension is used for compositing. Support for OpenGL 3/4 could also be added to this back-end that's compliant with the OpenGL core profile. However, it seems there isn't any interest in GL3/GL4 support in KWin at this point. That's a turn-around from originally there were plans for OpenGL 3.x support in KDE SC 4.7 although the free software graphics drivers are far from being ready.
The OpenGL ES 2.0 / EGL back-end is the primary target for mobile devices with Plasma Active, but it can be used on the desktop. It's also this back-end that's being designed to eventually work with the Wayland Display Server due to its support of EGL over GLX. The GLES 2.0 back-end is written again against the core OpenGL profile and uses GLSL shaders. With the EGL usage, EGL_KHR_image_pixmap is used over GLX_EXT_texture_from_pixmap. With KDE SC 4.7, the OpenGL ES back-end can only be configured at build-time, but for KDE SC 4.8 it may turn into being a run-time switch.
The X Render back-end can only be used by user opt-in from the KDE/KWin configuration options or when OpenGL support fails. Obviously, as this back-end is just targeting an X Server extension, it is only accelerating 2D animations assuming your DDX driver has X Render support. If the X Render back-end support fails, you fall-back to no compositing from KWin.
Martin's post in full with more details can be read on his blog.