To perform the multitasking operation the operating system provides processes and threads. But today will be going into more detail about what are the types of threads. There are mainly two types of thread one is the user-level thread and the other one is the kernel-level thread. As you can easily understand from the name of these threads that one thread is created by the user and the other thread is created by the kernel and therefore they are called respectively.
The user-level threads have their predefined library and these libraries make it very easy to call the threads but for the kernel-level thread they are created by the operating system and as it utilizes the operating system, therefore, the system calls become an integral part of the making of the kernel level threads. The user level threads are therefore faster because they use the predefined process and calls from the library and also the kernel level threads use system calls and these system calls are much time taking therefore it slows down the speed of the thread. And therefore, for this reason, only the user level threads are much faster than the kernel level threads.
As we know that the process is a more heavy task as compared to the thread which is a lightweight task because the thread has shared resources between each other they share their data their code therefore switching from one user level thread to another is just within a fraction of seconds but when the switching takes place in the Process then the first step is to store all the values in a process control block which will have the value of the address of register and the commands that they are following, therefore, this will consume a great amount of time.
If one user level thread is blocked then the other user level threads will also be blocked because as the threads are sharing their resources such as memory and register if you block one register for one single thread then the same register will be blocked for the other threads that were sharing the resource, therefore, the user level threads. Therefore if you block any resource of the parent threads then the child threads will be affected and vice versa. But in the case of kernel level thread and as these threads uses the operating system calls and function to use these threads and to create the copy of their codes, therefore, these kernel level threads will remain unaffected even if you block any one of the threads, unlike the user level threads where if you close one thread other threads will also be closed therefore these are the advantages of the kernel level thread that it will not be closed if you close one of its thread.
Now if you talk about which of these technologies is most used by the operating system that it is the hybrid of both the technique that it uses both the user level thread and the kernel level thread simultaneously and whenever there is a process of higher priority it has to be performed in any case then it is allocated the kernel level thread and if the process is a lower priority then they are allocated the user level thread. Also, the kernel level thread has a little disadvantage that it is much harder to create a kernel level thread, and also the kernel mode is often required to switch from one PCB to another PCB therefore these are some differences between the user level threads and the kernel level threads.