The "X" is simply a placeholder for "E", "F", "T", etc. and CXO is a shortened way to refer to someone at C-level. So a conference inviting the CXO is seeking anyone at the C-level within a company.
It's interesting to note that the C-level roles are fairly consistent globally, but below this level, corporate boards differ around the world. In most of North America, for example, it is not uncommon for a set of VPs to be members of the Board, along with the C-level roles. They have a strategic role and Directors report to them. In Europe, this is reversed and the Directors are often a part of the Board. There are also differences between for-profit and not-for-profit businesses.
The bottom line is that the governance of each corporation needs to be looked at individually to get the best picture of how they are run.