Extract gnutls library files of tar.xz and tar.lz

Question

I am very new to linux platform and I want to extract gnutls for ubuntu. If I do,

$ls

then, it will show these files below.

gnutls-3.2.1.tar.lz
gnutls-3.2.1.tar.lz.sig
gnutls-3.2.1.tar.xz
gnutls-3.2.1.tar.xz.sig

When I give command,

$ tar -xvf gnutls-3.2.1.tar.xz

I get output,

tar: xz: Cannot exec: No such file or directory
tar: Error is not recoverable: exiting now
tar: Child returned status 2
tar: Exiting with failure status due to previous errors

I want to extract these files.

1. Why there are two types of formats given? - xz and lz
2. Why there are signature files? - .sig How to verify it? I tried to verification by gpgv <filename>.sig command, it says public key is not found.
3. I need a separate utility package? I saw one, 'xz-utils_4.999' but it is beta. http://manpages.ubuntu.com/manpages/lucid/man1/xz.1.html
4. Any other stable utility package? From where and how can I download it?

Answer 1

For tar.lz, if your tar doesn't support --lzip

tar xf filename.tar.lz --use-compress-program lrunzip