Command xkill is used to kill a process on X server without passing process name or PID. It forces the X server to close the communication with its clients, which ultimately kill its clients by its X resource. In short, xkill instructs X server to terminate client.
To get a list of x clients, use command
Look at the above snapshot, this is the complete list of x clients in our system.
To use xkill command
Using xkill command when you want to kill a process, type xkill on the terminal. Your cursor will change in the shape of x, click on the window which you want to kill using x cursor. You'll get the following message as shown in below snapshot.