Compare user threads and kernel threads.
User threads:-
User threads are supported above the kernel and are executed by a thread library at the user level. Thread creation & scheduling are done in the user space, without kernel intervention. Thus they are fast to create and manage blocking system call will cause the whole process to block
Kernel threads:-
Kernel threads are supported directly by the operating system .Thread creation; scheduling and management are completed by the operating system. Thus they are slower to create & manage compared to user threads. If the thread performs a blocking system call, the kernel can schedule another thread in the application for implementation.