xcp的全称是， xen cloud platfom, 从名字上看，这些软件都和xen有关系，实际上也如此．
这些都是虚拟化的相关产品．xen是一个Hypervisor, 所谓的vmm(virtual machine manager)管理程序，是硬件虚拟化的众多技术之一，其目的就是为了让多个操作系统同时运行在一台主机设备．我们可以将上面描述的环境(架构)叫做云计算中的iaas. 可以使共有云,或者私有云.
Robert P. Goldberg 将Hypervisor分为2种类型:
type1: 例如有Citrix XenServer, VMware ESX/ESXi, and Microsoft Hyper-V (vm运行在硬件上的第二层)
type2: KVM and VirtualBox are examples of Type 2 hypervisors. 这里我认为vmware workstation, fusion 也应该属于type2. (vm运行在硬件上的第三层)
至于xen我认为也应该属于type1, 因为xen server 和 xcp都是基于xen衍生过来, 而 xen仅仅是一个简单的Hypervisor, 而 citrix xen server , xcp 是一个hypervisor + api + tools 的套装.
said by Scott Alan Miller on community.spiceworks
Xen: The open source, completely free granddaddy of the Xen family (which at one time had many children, most of whom are now gone.) Xen is different from all of its competitors in that it is a hypervisor ONLY and not a virtualization ecosystem (no management console, no extra features, no nothing.) It’s just a tiny little hypervisor. This is confusing because we lump Xen with HyperV, for example, but it is not an apples to apples comparison. Xen is like the hypervisor inside HyperV that no one even knows the name of.
XenServer: The primary commercial virtualization suite built on technologies derived from Xen. XenServer is an “older” version of Xen with some features removed and other featurs hidden behind “add ons.” So XenServer is far less functional than Xen and is not completely free (it can be, but many features are not.) XenServer is a full suite with all of the nice console and tools, APIs, etc. Much more user friendly than Xen and requires no core OS knowledge to get it working unlike Xen that requires you to really know the Dom0 environment well.
XCP: Xen Cloud Platform. XCP is a full virtualization suite based on Xen. It uses real Xen and combines in all of the other components needed to be a full suite just like XenServer, vSphere and HyperV. It is open source and comes from the Xen people. Today, when people talk about Xen, they mostly talk about XCP as XCP is the apple to apple with the other products and not just the hypervisor. You lose no functionality going with XCP over Xen on its own but it is far easier. XCP is designed to be interoperable with most, if not all, APIs from XenServer so that they can share tools giving XCP a lot of important options.