nvd, diff the latest 2 home-manager generations:
home-manager generations | head -n 2 | cut -d' ' -f 7 | tac | xargs nvd diff
Of course only gets you the changes after they are installed.
This is also built-in now using
nix store diff-closures:
$ nix store diff-closures /nix/store/xfy75lrmsh23hj2c8kzqr4n1cfvzh1s2-home-manager-generation /nix/store/5rdyvvk6jngd2hd44bsa14bpzxraigbi-home-manager-generation gnumake: 4.4.1 → ∅, -1546.9 KiB home-manager: -9.0 KiB
via Nix Manual: nix store diff-closures