Virtual CPUs in XenServer are similar to vCPUs in VMware. There is a resource sharing algorithm that ensures that virtual machines get their fair share of the host resources, specifically the CPUs.
Assigning a virtual machine's vCPUs to specific physical CPUs in XenServer is referred to as CPU pinning. The vCPU priority weight can be used to grant a virtual machine more CPU time than other virtual machines. The CPU cap fixes the amount of CPU that a virtual machine can use.
As in VMware, changing these values from the defaults isn't really necessary and can actually limit your virtual machines from taking advantage of resource optimizations implemented by the XenServer hypervisor.