virt-xml-validate: 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-xml-validate - from package: libvirt-client

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

On Debian possible default full paths are:

/usr/bin/virt-xml-validate - from package: libvirt-clients

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

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