virt-diff: 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 Debian possible default full paths are:

/usr/bin/virt-diff - from package: libguestfs-tools

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

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