I have never used Hyper-V, but am nevertheless trying to help.
To be sure we are talking about the same thing: You fire up Windows XP in Hyper-V and then, immediately after logon, you get a screen which tells you that Windows must be activated, and your virtualized system (apparently) does not let you do anything else.
In that "registration screen", there should be three options, among them an option like that (I can't remember the exact wording): "Telephone service to activate Windows" or perhaps "Activate Windows by phone" or such. If you can see this option, then why not try it? I have done that several times for customers, and in every case, the service representatives were very friendly and just asked if we did use that Windows copy on other PCs (of course, we didn't). Then they passed our call to some automated system which told us some activation code - problem solved. If you mistrust them, you could suppress your caller ID for that call ...
If you can't or don't want to activate by phone for some reason, you have to deploy a more advanced technique. The easiest way is tricking Windows into opening a file manager window. To do that:
1) In the "registration screen", there should be some help link.
2) Now, hoping that Internet Explorer is your standard browser, if you click on that help link, Internet Explorer will open (please tell us if Internet Explorer isn't your standard browser - I don't want to make it too complicated here).
3) When Internet Explorer has opened, clear its address bar (i.e. clear the current URL).
4) Then, in the blank address bar, type "C:\". Now a normal Windows File Explorer window should open. In other words: Using Internet Explorer, you just have opened Windows File Explorer.
Please note that this is not my invention. I have read about it several times a long time ago.
5) Connect a USB stick to your PC which contains the drivers for the network card which Hyper-V simulates, i.e. the drivers for the virtual network card Windows XP is seeing from within its VM, and make sure that the VM / Windows XP is seeing that USB stick (you may have to change your VM configuration for that, bus as as said above, I don't know anything about Hyper-V).
6) Using your precious File Explorer Window from step 4), install those drivers from the USB stick.
7) If you have a standard configuration (i.e. DHCP server somewhere), then chances are that the Internet works now. In that case, you can now do the activation via Internet. Otherwise, after having installed the drivers, you will have to do the network configuration first.
If you have a problem with one of the steps above, then describe the problem as precise as possible, including screenshots if possible, but at least telling us the literal error messages (if applicable).
Could you please clarify if you can log in normally to your XP system, i.e. if you can use it normally but it reminds you of registering / unlocking, or if you already have reached the state where it won't let you do anything but the registration, i.e. where the "registration screen" is the "shell" shown directly after logon and you can't start any other program? – Binarus – 2017-02-22T08:18:07.947
It is the latter. This was an old Windows XP mode disk I had which I guess I never got registered when I created it a long time ago – Tom – 2017-02-22T11:52:18.063
I don't remember having to activate XP mode back in the day. I would suspect the need for re-activation might be due to the hardware changing. – Journeyman Geek – 2017-02-25T11:25:57.797