Please read also: How to start synchronizing with booking portals?
For Booking.com - There is no XML password, nor any new password.
For Hostelbookers - Your password to your backoffice is your also your password to your xml connection in HostelSystem. You will just be notified that the connection is enabled, and you should use your backoffice password to enable connection.
For Hostelsclub - When the connection is enabled Hostelsclub will send you directly your new XML password. It is a different password from your backoffice one.
For Hostelworld - Once the connection is enabled, HostelSystem Team will send you your new XML password. It is a different password from your backoffice one.