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About XEN and KVM

KVM is a fully virtualized solution. Virtualization capabilities can be implemented on x86 architecture computers. KVM, however, requires support for virtualization in the CPU and can only run on CPUs with virtualization support, i.e. Intel CPUs with VT capability and AMD CPUs with AMD-V capability.

Xen is also a virtualization solution for Linux and will soon be incorporated into the kernel. Xen is implemented by running a kernel that supports Xen functionality. This kernel works under Xen's control and is called Domain0. After using this kernel to start the machine, you can use qemu software on this machine to virtualize multiple systems.

Xen vs KVM

KVM was developed by Israeli company Qumranet and has been incorporated into the Linux kernel by the Linux kernel organization, using x86 processors that include at least one of the hardware-assisted virtualization technologies (Intel-VT or AMD-V).

Xen is an external hypervisor; it can control virtual machines and allocate resources to multiple clients. After KVM is written to the Linux kernel, it is estimated that Xen is difficult to be written to the kernel; on the other hand, KVM is part of Linux and can use the usual Linux scheduler and memory management. This means KVM is smaller and easier to use.

In addition, Xen supports both full virtualization and para-virtualization (guest operating systems need to be modified for better performance), and KVM currently does not support para-virtualization.

The downside of Xen is that if you need to update the Xen version, you need to recompile the entire kernel, and, with the slightest carelessness, the system won't boot.

In comparison, KVM is much simpler. It does not require recompiling the kernel, nor does it require any modification to the current kernel, it is just a few dynamically loaded.ko modules. It has a simpler structure and smaller code. Therefore, the possibility of error is less. And in some ways, performance is superior to Xen.

1.Xen is superior to KVM in six major points:

(1) Available resources: Xen predates KVM by four years (2003 and 2007). As Citrix, Novell, Oracle, Sun, Ret Hat, and Virtual Iron implement in the marketplace, it becomes easier to find Xen-savvy IT technicians, get Xen training, get Xen consulting help, and get Xen certification. EMA's 2008 study of virtualization and management trends shows that these key factors account for 60% of businesses complaining about a lack of the necessary virtualization technology resources and technologies.

(2) Platform support: Xen supports more host and guest environments in the new release, including pan-virtualization technology, hardware-assisted support, and modified or unmodified guest operating systems; specific support for UNIX, Linux, and Microsoft Windows; chipset support for x86, IA64, AMD, Fujitsu, IBM, Sun, and others; and x86/64 CPU vendors and Intel embedded.

EMA's 2009 study on virtual systems management found that management was a critical or important factor in the choice of virtualization technology for 83% of enterprises. When comparing Xen to KVM, Xen has a broader community of third-party provisioning, backup, storage management, P2V, capacity planning, performance monitoring, process automation, security, and other management rules, such as Citrix, IBM, CA, Novell or Platepin, Enomaly, Microsoft, and HP.

(4) Implementation: Whether KVM is "class 1" or "class 2," it is a semantic concept. Xen is run and managed at a lower level (Ring 0). Even for new virtual machine creation, guest machines do not need to share memory blocks, CPU instructions, or any underlying Linux operating system (although occasionally override) as KVM does.

KVM does not support live migration: One of the most important arguments used to justify VMware ESX over Microsoft Hyper-V also applies to the comparison between Xen and KVM, but it's a big problem. Unlike KVM, Xen supports non-disruptive dynamic migration, allowing dynamic workload balancing and routine maintenance with virtually no downtime. KVM's inherent nature determines that it must have downtime.

(6) Performance: Most comparisons of Xen and KVM performance benchmarks show that Xen has better processing performance (close to native processing), only slightly worse than KVM in disk I/O. Further, independent testing has shown that KVM performance degrades as workloads increase. Typically, attempts to support more than four guest virtual machines will crash. Xen supports linear growth in the number of guest virtual machines and can support more than 30 workloads running simultaneously.

KVM outperforms Xen in Linux integration

Even without extensive Xen and KVM performance benchmarking, there are plenty of reasons why Linux leaders such as Red Hat and Ubuntu treat KVM. The most obvious and important factor is that KVM is part of the Linux kernel, and Xen is just a product installed under the Linux kernel.

Why is this so important? This is important because in the past patches for the Xen operating environment were not compatible with the Linux kernel. However, if KVM is implemented, this problem can be easily solved. Another reason for choosing KVM is that KVM is deployed inside the Linux kernel, making it easy to control the virtualization process.

Xen advocates claim KVM is less mature than Xen technology and lacks certain key features such as live migration and pan-virtualization support. Indeed, pan-virtualization in Xen workplaces can make virtual machines operate more efficiently because pan-virtualization interacts directly with hardware. However, using pan-virtualization technology requires operating system modifications, and the default Windows installation does not support pan-virtualization work environments. As for dynamic migration, KVM can also be done, but only if the correct KVM version is installed. KVM did have a problem with live migration in the past, but that has now been fixed.

In other words, KVM is more flexible. Because the operating system interacts directly with the virtualization hypervisor integrated into the Linux kernel, it can interact directly with hardware in any scenario without modifying the virtualized operating system. This is important because KVM can be a faster solution for running virtual machines. The fact that KVM requires Pacifica (AMD) or Vanderpool (Intel) virtualized CPUs is no longer a limiting condition for KVM development, as most server CPUs today have these processors.

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