virt-top: 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/virt-top - from package: virt-top

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

On Debian possible default full paths are:

/usr/bin/virt-top - from package: virt-top

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

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