By default, XenServer enables only Citrix yum repository. For example, to examine all enabled repositories on XenServer, run the following.
Loaded plugins: fastestmirror Loading mirror speeds from cached hostfile repo id repo name status citrix XenServer 5.6.100 updates enabled: 0
However, the default Citrix yum repository is void of any useful software. If you want to install a standard Linux package on XenServer, you need to enable a base repository as follows.
If you want to enable base repo on XenServer permanently by default, you can update /etc/yum.repos.d/CentOS-Base.repo as follows.
From this point on, you can install any software package available in base repository without explicitly enabling it.
$ sudo yum install gcc