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Xen
Xen is a hypervisor. It provides the fundamental isolation between virtual machines. Xen is open source (GPLv2) and is managed by Xen.org, an cross-industry organization. Xen is a component of many different products and projects. The hypervisor itself is very similar across all these projects, but the way that it is managed can be different, which can cause confusion if you're not clear which toolstack you are using. Make sure you know what toolstack you want before you get started.
Xen 是一个开放源代码虚拟机监视程序（VMM），由剑桥大学开发。它是一种开源 (GPLv2) 管理程序，现在已有 50 多家解决方案提供商为其提供支持。Xen 是一种可在 XenServer、Xen 云平台和 Oracle 虚拟机等其他发行版中使用的虚拟机管理程序。该虚拟机管理程序已由 Amazon Web 服务以及众多的 Web 托管服务和数据中心解决方案提供商使用。XEN 是一个基于X86架构、发展最快、性能最稳定、占用资源最少的开源虚拟化技术。Xen可以在一套物理硬件上安全的执行多个虚拟机，与 Linux 是一个完美的开源组合，Novell SUSE Linux Enterprise Server 最先采用了XEN虚拟技术。Xen虚拟机可以在不停止的情况下在多个物理主机之间实时迁移。在操作过程中，虚拟机在没有停止工作的情况下内存被反复的复制到目标机器。虚拟机在最终目的地开始执行之前，会有一次60-300毫秒的非常短暂的暂停以执行最终的同步化，给人无缝迁移的感觉。类似的技术被用来暂停一台正在运行的虚拟机到磁盘，并切换到另外一台，第一台虚拟机在以后可以恢复。它特别适用于服务器应用整合，可有效节省运营成本，提高设备利用率，最大化利用数据中心的IT基础架构。
总的来说，Xen 是一个极小的程序，大约由 150,000 行代码组成，可创建一个用来管理来宾操作系统和系统资源的虚拟机管理程序，其管理方式与传统操作系统中的管理程序非常相似。
Citrix XenServer
Citrix XenServer is a commercial product. It is based on XCP, and exposes the same toolstack and managment API. As an analogy, think of XenServer being based on XCP in the way that Red Hat Enterprise Linux is based on Fedora. XenServer has a free version (which is very similar to XCP) and paid-for versions with additional features enabled. 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.
XenServer是商用产品。Xenserver之于XCP就像RHEL之于Fedora。Xserver是一个拥有完整的控制台、工具栈及API接口的套件。XenServer比Xen更容易使用，XenServer包含了Xen，linux以及一个主控制守护进程（xapi）。
Xen 云平台 (XCP)
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.
2009 年，Xen.org 宣布启动 Xen 云平台 (XCP) 计划。根本而言，XCP 是 XenServer Free（免费版）的开源“对等物”，但是其中添加了部分仅 XenServer Advanced、Enterprise 和 Platinum 提供的功能（参见https://wiki.xenserver.org/XCP/XenServer_Feature_Matrix）
| Features | Xen Cloud Platform | XenServer Free | XenServer Advanced | XenServer Enterprise | XenServer Platinum |
| Cost/Licensing | Free/Open Source (Multiple Licenses1) | Free/Citrix EULA | Paid/Citrix EULA | ||
| XenProject hypervisor | X | X | X | X | X |
| IntelliCache | X | X | X | X | X |
| Resilient distributed management architecture | X | X | X | X | X |
| VM disk snapshot and revert | X | X | X | X | X |
| XenCenter management | X | X | X | X | X |
| Conversion tools | X | X | X | X | X |
| XenMotion® live Migration | X | X | X | X | X |
| Heterogeneous pools | X | X | X | X | |
| Dynamic Memory Control | X | X | X | X | |
| Performance alerting and reporting | X | X | X | X | |
| Distributed virtual switching management tool | X | X | X | ||
| High availability | X | X | X | ||
| Automated VM protection and recovery | X | X 3 | X 3 | X | |
| Host power management | X | X | X | ||
| Live memory snapshot and revert | X | X | X | ||
| Role-based administration | X | X | X | ||
| Dynamic workload balancing | X | X | |||
| Provisioning services (virtual) | X | X | |||
| StorageLink | X | X | |||
| Web self-service with delegated admin | X 2 | X | X | ||
| Site recovery | X | X | |||
| Lab manager with self-service portal | X | ||||
| Provisioning services (physical) | X | ||||
XenServer features taken from the XenServer Editions page
| [1] LGPL, GPL, Q Public License v1 |
| [2] works flawless using original Virtual Appliance from Citrix (tested: XCP 1.5beta (1.4.90) and XenServer-wss-1.1.1.xva) |
| [3] Automated VM protection and recovery is only available for the Advanced and Enterprise editions in the 6.0 release and later. |
Both XenServer and XCP include Xen, Linux, and the primary control daemon known as xapi.
The API shared between XCP and XenServer is called "XenAPI". OpenStack usually refers to XenAPI, to indicate that the integration works equally well on XCP and XenServer. Sometimes, a careless person will refer to XenServer specifically, but you can be reasonably confident that anything that works onXenServer will also work on the latest version of XCP.
| Description | Xen | XCP | XCP-XAPI packages in Linux |
|---|---|---|---|
| Hypervisor | latest | lagging | as in latest Linux distro |
| Dom0 OS | See Dom0_Kernels_for_Xen | CentOS (5.7 in XCP 1.6) | See Debian, Ubuntu and Fedora |
| Dom0 | 32 and 64 bits | 32 bits | 32 and 64 bits |
| Linux 3 PVOPS Dom0 | Yes | No | Yes |
| Toolstack | XM (deprecated), XL, libvirt | XAPI with XE | Same as XCP |
| Features | Xen Feature List | Xen Feature List and XCP Feature List | Xen Feature List and XCP Feature List |
| Storage, Network, Drivers | Build and get yourself | Integrated with Open vSwitch, multiple storage types & drivers | Get them yourself (only core components are provided) |
| Configurations | Everything | Constrained by XAPI | Same as XCP |
| Usage Model | Do it yourself | Shrink wrapped and tested | Do it yourself |
| Distribution | Source code or via host Linux or Unix distributions | Delivered as ISO on xen.org | Via host Linux or Unix distributions |
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