For up to date installation guide and other informations check libvirt provider's GitHub.
Create the following directory:
mkdir -p ~/.local/share/terraform/plugins/registry.terraform.io/dmacvicar/libvirt/0.6.2/linux_amd64
Move to created directory:
cd ~/.local/share/terraform/plugins/registry.terraform.io/dmacvicar/libvirt/0.6.2/linux_amd64
Download libvirt provider .tar.gz archive file
(all possible assets can be found under their releases):
wget https://github.com/dmacvicar/terraform-provider-libvirt/releases/download/v0.6.2/terraform-provider-libvirt-0.6.2+git.1585292411.8cbe9ad0.Ubuntu_18.04.amd64.tar.gz
Unarchive file:
tar -xzf terraform-provider-libvirt-0.6.2+git.1585292411.8cbe9ad0.Ubuntu_18.04.amd64.tar.gz
That's it.