Difference between KVM and LXC - linux-kernel

Question

What is the difference between KVM and Linux Containers (LXCs)? To me it seems, that LXC is also a way of creating multiple VMs within the same kernel if we use both "namespaces" and "control groups" features of kernel.

Answer 1

Text from https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/7-Beta/html/Resource_Management_and_Linux_Containers_Guide/sec-Linux_Containers_Compared_to_KVM_Virtualization.html Copyright © 2014 Red Hat, Inc.:
Linux Containers Compared to KVM Virtualization
The main difference between the KVM virtualization and Linux
Containers is that virtual machines require a separate kernel instance
to run on, while containers can be deployed from the host operating
system. This significantly reduces the complexity of container
creation and maintenance. Also, the reduced overhead lets you create a
large number of containers with faster startup and shutdown speeds.
Both Linux Containers and KVM virtualization have certain advantages
and drawbacks that influence the use cases in which these technologies
are typically applied:
KVM virtualization
KVM virtualization lets you boot full operating systems of different
kinds, even non-Linux systems. However, a complex setup is sometimes
needed. Virtual machines are resource-intensive so you can run only a
limited number of them on your host machine.
Running separate kernel instances generally means better separation
and security. If one of the kernels terminates unexpectedly, it does
not disable the whole system. On the other hand, this isolation makes
it harder for virtual machines to communicate with the rest of the
system, and therefore several interpretation mechanisms must be used.
Guest virtual machine is isolated from host changes, which lets you
run different versions of the same application on the host and virtual
machine. KVM also provides many useful features such as live
migration. For more information on these capabilities, see Red Hat
Enterprise Linux 7 Virtualization Deployment and Administration Guide.
Linux Containers:
The current version of Linux Containers is designed primarily to
support isolation of one or more applications, with plans to implement
full OS containers in the near future. You can create or destroy
containers very easily and they are convenient to maintain.
System-wide changes are visible in each container. For example, if you
upgrade an application on the host machine, this change will apply to
all sandboxes that run instances of this application.
Since containers are lightweight, a large number of them can run
simultaneously on a host machine. The theoretical maximum is 6000
containers and 12,000 bind mounts of root file system directories.
Also, containers are faster to create and have low startup times.
source

Answer 2

LXC, or Linux Containers are the lightweight and portable OS based virtualization units which share the base operating system's kernel, but at same time act as an isolated environments with its own filesystem, processes and TCP/IP stack. They can be compared to Solaris Zones or Jails on FreeBSD. As there is no virtualization overhead they perform much better then virtual machines.
KVM represents the virtualization capabilities built in the own Linux kernel. As already stated in the previous answers, it's the hypervisor of type 2, i.e. it's not running on a bare metal.

Answer 3

This whitepaper gives the difference between the hypervisor and linux containers and also some history behind the containers
http://sp.parallels.com/fileadmin/media/hcap/pcs/documents/ParCloudStorage_Mini_WP_EN_042014.pdf
An excerpt from the paper:
a hypervisor works by having the host operating
system emulate machine
hardware and then bringing up
other virtual machines (VMs)
as guest operating systems on
top of that hardware. This
means that the communication
between guest and host
operating systems must follow
a hardware paradigm (anything
that can be done in hardware
can be done by the host to the
guest).
On the other hand,
container virtualization (shown
in figure 2), is virtualization at
the operating system level,
instead of the hardware level.
So each of the guest operating
systems shares the same kernel, and
sometimes parts of the operating system, with
the host. This enhanced sharing gives
containers a great advantage in that they are
leaner and smaller than hypervisor guests,
simply because they're sharing much more of
the pieces with the host. It also gives them the
huge advantage that the guest kernel is much
more efficient about sharing resources
between containers, because it sees the
containers as simply resources to be
managed.
An example:
Container 1 and Container 2 open the same file, the host kernel opens the file and
puts pages from it into the kernel page cache. These pages are then handed out to
Container 1 and Container 2 as they are needed, and if both want to read the same
position, they both get the same page.
In the case of VM1 and VM2 doing the same thing, the host opens the file (creating
pages in the host page cache) but then each of the kernels in VM1 and VM2 does
the same thing, meaning if VM1 and VM2 read the same file, there are now three
separate pages (one in the page caches of the host, VM1 and VM2 kernels) simply
because they cannot share the page in the same way a container can. This
advanced sharing of containers means that the density (number of containers of
Virtual Machines you can run on the system) is up to three times higher in the
container case as with the Hypervisor case.
Summary:
KVM is a Hypervisor based on emulating virtual hardware. Containers, on the other hand, are based on shared operating systems and is skinnier. But this poses a limitation on the containers that that we are using a single shared kernel and hence cant run Windows and Linux on the same shared hardware