In earlier times, X was used to seal a letter (or other documents of confidence) as a symbol of faith/trust. It is understood that was a mark of a kiss...kissing to seal a document.

O was added to X much later in modern times as a symbol of a hug...

XO, therefore, stands for a kiss and a hug...XOXO being in the plural.

So, XOXO to all! Happy Valentines!