List the three common ways of mapping user threads to kernel threads.

1. Many-to-one mapping: In this approach, multiple user-level threads are mapped onto a single kernel-level thread. The user-level thread management is handled entirely in user space, and the kernel has no knowledge of the user-level threads. This approach is simple to implement, but it suffers from poor performance and does not exploit the full potential of multi-core systems.

2. One-to-one mapping: In this approach, each user-level thread is mapped to a separate kernel-level thread. This approach provides better performance and scalability than the many-to-one approach, but it requires more system resources and incurs more overhead due to context switching.

3. Many-to-many mapping: This approach is a hybrid of the above two approaches. In this approach, multiple user-level threads are mapped onto an equal or smaller number of kernel-level threads. The user-level threads are managed in user space, and the kernel-level threads are managed in kernel space. This approach provides a balance between performance and resource utilization.