It seems to be possible to classify kernels into three different types. Firstly, there are the monolithic microkernels where all services are provided by the kernel and there is a big distinction between kernel and non-kernel systems. Kernels are traditionally non-pageable for engineering reasons and thus desirable additional features, such as a filing system based on the ftp protocol do not get added as this would lead to unacceptable memory usage by the kernel.
Once we have the concept of a microkernel, the obvious approach is to run an operating system emulator on top of it. Effectively, the microkernel provides a virtual processor which the emulator runs on. This design is called a single-server and these systems form the second category. The microkernel will not be pageable and the single-server may or may not be. It is still monolithic to a great degree however and it will still be very difficult to add functionality to it. Given this base, it is now feasible to run and debug alternative versions of the single-server concurrently with the base one, allowing for faster, more convenient prototyping.
Thirdly, we can take the multiserver approach and split the single-server into a collection of servers. This allows for some servers to be paged out if they are not currently being used while others remain paged in. If one server contains bugs, its ability to affect other tasks is greatly diminished and the problem can be isolated and dealt with much more efficiently. Again, replacement servers can be developed concurrently with a `production' collection of servers handling everyday usage.