I have problem in understand kernel threads ..what actually is kernel threads?

Short answer to your question: CPU only runs kernel thread, because kernel sees only kernel threads and the kernel manages CPU scheduling. If a user process wants to performance a task, one of the user process threads would be mapped to a kernel thread so it can be scheduled by kernel and ran by CPU. Programmers do not need to worry about mapping the user threads to kernel threads.

Maybe in some systems, programmer could bind a user thread to a kernel thread, but I am not sure if this is possible.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.