The difference between the EXT version of a Kernel and the non-EXT version of a Kernel, is simply down to the version of the compiler and compilation Operating System used by SAP to compile the Kernel binaries.
As an example, the 7.21 kernel could be compiled on Windows 2003 Server, using the Visual Studio 2010 compiler.
The 7.20EXT kernel could be compiled on Windows 2003 Server, using the Visual Studio 2012 compiler.
The difference is all about the compilation environment, and nothing to do with functionality. Or is it...
If you look at SAP note 1926209 - "Questions on subjects of 7.20/7.21 EXT kernel and C/C++ runtime", this would seem to be the case.
However, read SAP notes 1756204 - "Activate fast polling" and 1728283 - "SAP Kernel 721: General Information" , you will see that it seems to suggest that SAP can and will change the functionality between an EXT and non-EXT kernel (7.21 is used as the example here).
So, be wary and always read up about the benefits of each Kernel, whether EXT or not.