Citrix XenServer is an open source server virtualization platform based on the Xen hypervisor. Citrix also offers a supported version that you can purchase, with two options: Standard and Enterprise.
What XenServer is used for
This platform is used by virtualization administrators to deploy, host and manage VMs. It's also used to distribute hardware resources -- CPU, memory, networking, storage -- to VMs.
Key features of Citrix XenServer aim to ease virtualization infrastructure management. VM templates are a significant aspect of this. For example, you can create VM templates from snapshots. Another key feature of this platform is XenMotion, which allows you to live migrate VMs between hosts. With the Enterprise version of XenServer 7.1, you can also live patch hosts with no downtime. Centralized support for Open vSwitch is another important feature of the XenServer platform.
XenServer management console
The XenServer management console, which is a Windows-based client, sets this platform apart from other Xen stacks because it gives you the ability to manage VMs running on multiple hosts in a resource pool. Using this enterprise-level management console, you can install, provision and run VMs on a XenServer host. Admins can also configure a remote storage repository for VMs.
The most recent Citrix XenServer update is 7.2, which was released on May 25, 2017. XenServer 7.2 touts better integration with XenApp and XenDesktop. Added support for Server Message Block storage protocol is also included.
XenServer 7.0 was released on May 24, 2016, with a higher-end graphics ability, new features and security capabilities. Citrix increased the amount of NVIDIA vGPU-enabled VMs that XenServer can support. The company also increased host RAM, CPU cores and VM RAM scalability support.
XenServer 6.5 was released on Jan. 13, 2015, offering support for Docker. Released on Sept. 30, 2011, XenServer 6.0 focused on cross-platform management. XenServer 5.5, which was released on June 16, 2009, included Active Directory integration. Citrix introduced high availability (HA) in XenServer 5.0, which was released on Sept. 15, 2008.
Pros and cons
Critics of XenServer argue that its ecosystem isn't as established as those of its major competitors, but the fact that there is an open source version of this hypervisor makes it a good choice for smaller IT shops. Another drawback of Citrix XenServer is that it lacks built-in automation capabilities. In order to automate tasks, you have to use PowerShell scripts. In addition, compared to its competitors, XenServer doesn't have the same level of integration or available certifications.
The main architectural components of Citrix XenServer are the 64-bit hypervisor platform, XenCenter integrated management and XenMotion live migration. The XenServer installation program is essentially that used by previous versions of Red Hat Enterprise Linux (RHEL) with certain Citrix specifications.
Starting with Citrix XenServer 7, the Enterprise version of this platform includes Direct Inspect APIs, which allow third-party vendors to secure the OS, protecting against viruses, malware and rootkits. The Enterprise version also includes the Measured Boot feature, which protects the hypervisor, and Secure Tags, which ensure VMs only run on approved hardware.
Citrix XenServer's two main competitors are VMware vSphere and Microsoft Hyper-V.
XenServer vs. VMware vSphere
A knock against vSphere is its price point for advanced features, but it does have a lot of third-party management tools, applications and utilities that XenServer lacks. Training admins on vSphere and obtaining certification can be challenging because vSphere's advanced functionality and features make it a more complex platform. Security is similar in both, as VMware offers vShield, which compares to Citrix's Direct Inspect APIs.
XenServer vs. Microsoft Hyper-V
One point for Hyper-V is that a lot of companies already use Microsoft products and have familiarity with them. XenServer is a lower-cost option compared to Hyper-V and is a good fit for companies looking for desktop virtualization, application virtualization and/or a hyper-converged infrastructure.
XenServer backup and recovery
Citrix XenServer backup and recovery is made easier with VM templates and snapshots. Also, both the Standard and Enterprise versions include HA, which enables localized VM recovery in the event of a failure, and Site Recovery Manager, which allows admins to recover VMs from a major hardware failure in the CPU, networking and/or storage layers.
At its core is the Linux Foundation's Xen Project, which might not be the virtualization leader it once was, especially when you look at the increasing popularity of KVM -- Red Hat actually dropped Xen support in RHEL 6 in favor of KVM -- but it is gaining more traction with cloud providers.
In terms of Citrix XenServer specifically, while it's on par with VMware vSphere and Microsoft Hyper-V from a technical standpoint, its competitors have more robust ecosystems and, as a result, a stronger customer foothold. XenServer -- and Citrix in general -- is a good fit for niche use cases and will most likely continue to make headway in that space.