xzdiff: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/xzdiff - from package: xz

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/xzdiff - from package: xz-utils

You may install the required package with command apt-get install {package}

Error in other languages:
xzdiff: bevel nie gevind nie
xzdiff: comandă negăsită
xzdiff: comando não encontrado
xzdiff: commande introuvable
xzdiff: command not found
xzdiff: käsku ei ole
xzdiff: không tìm thấy lệnh
xzdiff: komanda nerasta
xzdiff: Kommando nicht gefunden.
xzdiff: kommandot finns inte
xzdiff: komut yok
xzdiff: nie znaleziono polecenia
xzdiff: níor aimsíodh an t-ordú
xzdiff: no se encontró la orden
xzdiff: no s'ha trobat l'ordre
xzdiff: opdracht niet gevonden
xzdiff: parancs nem található
xzdiff: perintah tidak ditemukan
xzdiff: príkaz nenájdený
xzdiff: příkaz nenalezen
xzdiff: команда не найдена
xzdiff: командата не е открита
xzdiff: 命令找不到
xzdiff: コマンドが見つかりません