Windows VM guest How To Install XenServer Tools ( Citrix Tools for Virtual Machines )

When I was a poor newbie with XenServer (until yesterday :-) ), I had to search a lot to find the instructions on how to install xen-tools for xenserver ;-P

The main error was searching for xentools or xen-tools, that is not the correct name for Citrix XenServer.
The correct one is Citrix Tools for Virtual Machines, what they at Citrix call
The Citrix paravirtualized network and SCSI drivers (Citrix Tools for Virtual Machines)

If you experience XenServer poor performances in Windows VM guests disk write speed or network file transfer speed, you'd better read the full story about how to install and purposes of
Citrix Tools for Virtual Machines formerly PV (paravirtualized) device drivers
(click the link to read official documentation from Citrix)

Please note that official documentation says:

Running Windows VMs without these drivers is not supported.

Please also note that official documentation says:

Warning:
Before updating Windows operating systems you must uninstall the PV device drivers. If they are present during the attempt to update, the update will fail.

Ciao
R.