As you might know Windows 8 Task Manager redesigned to suit Windows 8 UI, the new Task Manager offers more information about the running processes, applications and background services than old Task Manager in previous versions of Windows. You should know taskmgr.exe still opens old Task Manager in Windows 8 and TM.exe opens new Task Manager.
TM.exe is new Windows 8 task Manager
Open the task manager in WDP by right-clicking and clicking “Task Manager “ on the taskbar (use Ctrl +Shift +Esc shortcut instead). Task Manager will be shown under list of Applications under Processes tab. Right-click on it and click “Go to Details” to see TM.exe process running for Windows 8 Task manager, you can also observe task manager.exe process also running there.
For the convenience of users switched from Windows 7, Microsoft seem to kept old task manager for easier access to running process and applications in Windows 8. Of Course you can compare this scenario with Desktop and Metro Control Panels, Both Tm.exe and taskmgr.exe can be found System 32 folder in Windows directory.