User and Kernel level Threads:

Kernel Thread executes in kernel mode. and user-level thread executes in user mode.

OS has control of Kernal mode and user have control on user mode.

How user mode communicates with kernel mode?

User mode communicates with kernel mode with the help of system calls. System calls are built in functions in OS. 

kernel level thread	User level thread
Recognized by the operating system.	Does not recognized by the operating system.
Implemented by the operating system.	Implemented by a user of the system.
Implementation is complex.	Implementation is simple and easy.
Solaris is an example	Posix is an example
Requires hardware support	Requires no hardware support