Kubernetes supports 3 minor release concurrently. This means if the latest Kubernetes release is v1.19, that v1.18 and v1.17 will remain supported for backport fixes but that v1.16 will lose support.
Similarly, each KubeVirt release maintains compatibility with the latest 3 Kubernetes releases that are out at the time the KubeVirt release is made.
Here are some theoretical examples (note that the versions are made up to illustrate the example)
KubeVirt release v0.1 is cut. At that point in time the latest Kubernetes version is v1.3. This means KubeVirt v0.1 will forever be compatible with Kubernetes v1.3, v1.2, and v1.1.
KubeVirt main always follows the latest 3 Kubernetes releases. If a new Kubernetes v1.4 release is cut, that means support for v1.1 will be dropped for KubeVirt main.
Note that this support for the latest Kubernetes releases doesn't happen immediately. There is a period of time, usually a few weeks, where the new version of Kubernetes must be integrated into KubeVirt's CI. The goal here is to integrate the latest Kubernetes version into CI before the next KubeVirt release is cut.
KubeVirt main supports Kubernetes releases v1.4, v1.3, and v1.2. However, the KubeVirt v0.1 release was cut when the latest Kubernetes release was v1.3.
This means that KubeVirt v0.1 supports Kubernetes v1.3, v1.2, v1.1 while KubeVirt main is tracking support for Kubernetes v1.4, v1.3, v1.2.
The KubeVirt community maintains the ability to extend support for a KubeVirt release to older and newer Kubernetes versions that may fall outside of the latest 3 Kubernetes release present at the time of the KubeVirt release.
The KubeVirt commitment to supporting the latest 3 Kubernetes release is a commitment that this is the minimum guarantee we support and test. We will not support less than this minimum, however there are times when we may choose to support more Kubernetes releases than our minimum guideline suggests.
It is possible, and likely, that KubeVirt releases may continue to work with unsupported Kubernetes versions. Understand that we make no guarantees for this support. While it may technically work, there is no testing or CI being performed to guarantee it remains working with future KubeVirt patch releases.