Below just a few resources I have collected related to Windows and Office license activation with XenDesktop. Licensing is not an exciting topic but we have to deal with it anyway.
KMS Configuration using Citrix Provisioning Services
Citrix Managing Microsoft KMS Volume Licensing
Citrix Configuring Microsoft MAK Volume Licensing
Citrix XenDesktop 5 Support Statement for Microsoft KMS activation with Machine Creation Services (MCS)
Licensing Windows for VDI environments