I'm going to get a new server probably with OS Win2008 Server. That server will run some VM. Some with Linux, others with Windows.
Grant the Win2008 server license similiar rights to run e.g. Win2003 Server virtual instances, as Win7 desktop does with WinXP ?
No there is no similar feature as the one provided by Win 7. You'll have to run Hyper V or some other tye of hypervisor
The Windows 2008 Server equivalent to X-Mode is Hyper-V, if you want to run Win2003 Server you need it's own license, which you may already have from the past.
Some versions of Windows Server 2008 allow you to run some virtual instances of 'Windows Server 2008' in Hyper-V, how many depends on the version:
I think this is similar in 2008R2, there you can run VMs with 2008R2
You can also run HyperV Server as a "core/headless" type install. http://www.microsoft.com/en-us/server-cloud/hyper-v-server/