Extensions are Device Drivers ?

I havn't done any symbian kernel development, so I can't say I know the lingo to 100% but as I see it:

A Device Driver is always a Kernel Extension.
A Device Driver also is an adaptation for some hardware device (or emulates a hardware device in sw)
But.. you could want to extend the kernel without it being any adaptation for some specific hardware device.
In that case, you would write a Kernel Extension that is not a Device Driver...
So a Kernel Extension is not always a Device Driver.

I also expect that people mix the two up in daily speak...

Extensions are merely device drivers that the kernel automatically starts at boot-time, so you can think of them as a way to extend the kernel’s
functionality. For example, the crash debugger is a kernel extension,allowing you to include it or exclude it from a ROM as you wish, without having to recompile the kernel.

Hope it's useful for you to clear it