There are several ways to configure a Network File Share (NFS) on a Windows Server 2008 R2 so that a Unix system (such as XenServer) can connect to it.
CTX128116 – How to Use a Windows 2008 Network File Share as a Storage Repository for XenServer describes how to use anonymous access to the share. If you prefer not to give anonymous access to it, this article explains other options available.
In Windows Server 2003 R2, services for Unix, include User Name Mapping. This feature allows the mapping of Unix accounts to Lightweight Directory Access Protocol (LDAP) user accounts, which effectively enables standard Kerberos authentication methods from Unix systems (see http://technet.microsoft.com/en-us/library/cc779831%28WS.10%29.aspx).
In Windows Server 2008 R2, this feature has been removed. The following Microsoft document explains other available options: