My computer runs Windows Server 2008 R2. It hosts a Hyper-V virtual machine running Ubuntu 12.04 as the guest OS.
I want to copy text from Ubuntu and paste this text in Windows (and copy text in Windows and paste it in Ubuntu). How can I do this?
You can use
ncat - which also has a windows port - to transfer data over network. On one system you run it in "listen" mode, where it binds to some port, on the other system you connect to that port on the other machine. This creates a bi-directional pipe. On Linux you can choose from more variants (GNU netcat, BSD netcat, socat...). Apart from the obvious man page, you can also have a look at the wikipedia netcat article.
Note: on both systems you run these in the terminal (Windows command line, Unix shell) - the copy-paste has to happen twice: on one machine you copy from the source and paste it into the terminal which is running ncat. The data is transferred to the other machine, where you copy it from the terminal to its final destination.
Other option is to exchange files over a Windows share (Samba on Linux).
It depends on how you are accessing the guest from the host. Is it the console of the guest through a HyperV connection manager? HyperV has a feature in the Connection Manager to "Type Clipboard Text".
Does the guest run X? There appears to be no native copy/paste functionality between HyperV and X, so you may want to run a remote session using something like VNC or perhaps NX. And aside from the Type Clipboard Text, it looks like you cannot copy on the guest or read from its clipboard.