>how can i know what is default Kernel version

By reading the release notes. With every new release, the development team publishes the major changes compared to the previous releases. The kernel version is mentioned under the Key Features section.

The Release Note for [8.1-r4][1] indicates:
> - Update to LTS kernel 4.19.110.

For [8.1-r5][1]:
> - Update to LTS kernel 4.19.122.

For [9.0-r2][3]:
> Update to LTS kernel 4.19.110.

[1]: https://www.android-x86.org/releases/releasenote-8-1-r4.html
[2]: https://www.android-x86.org/releases/releasenote-8-1-r5.html
[3]: https://www.android-x86.org/releases/releasenote-9-0-r2.html