If I'm understanding you correctly, OS X would be the OS and XP would be run using Parallels/Fusion.
When XP runs, it is basically "running in a window" just like any other application. You can resize it, minimize it, etc. The OS X desktop will still be visible behind it. The exception to this is when you choose to enter full-screen mode. In full screen, you will not be able to tell OS X is installed on the Mac be looking at the display. Windows takes up every pixel on the display.
If you shut down XP, it doesn't turn off the computer. Windows logs off and the window that XP was running in closes (like when you quit any other application). OS X is still running and the desktop is still there ready to use.
Hope this makes sense. I guess you can't think of XP as an OS while it is running in a VM. It's more like an application. You open it, use it, and then close it when you are finished--all the while OS X (the OS) never stops running.
I think I understand, starting XP will be like any other program, so it doesn't go through the whole startup routine. But I guess it still installs all the video drivers etc, that XP needs.
You basically open the virtual environment application (vmware, parallels etc..) and start windows xp. Windows XP will then go through the same boot sequence it does when you start a PC.
To shut it down, its basically the same.. Start > Shutdown. Once windows xp has shut down you can close the virtual environment application.
You can allocate how much memory is dedicated to XP. I only use it occasionally and usually only have a single app open in it. Of my 2gb, 512mb is dedicated to XP (only when it's open) and the balance remains with OS X. If you had 4gb you could allocate more if necessary.
This is one of the reasons many prefer XP over Vista in a VM. You want to use a system which requires the least amount of resources to do what you need it to do.
Yes. It's not like partitioning a HD. You are just telling the VM how much memory it's allowed to use when it opens. It's a setting in the preferences.