European Spanish
Latin American Spanish
In Spain, the letter “j” carries the sound [x], a voiceless velar fricative or "j" (jarro), "g" (general). In parts of Latin America and southern Spain, the letter “j” is pronounced [h]. In parts of South America and Southern Spain it is also not pronounced in final position (/reloj/: [relo])
The letter “X”, however, varies even more in sound, depending on the origin of the word.
In the words with the letter “x” (like 'excelente') pronunciation varies between Spain and Latin America. In Spain the 'x' is usually pronounced as an [s] ('esselente' or 'esthelente'), while in most of Latin America it is pronounced similar to English [ks] ('ekselente').
In Latin America, the letter “x” is pronounced as [s] at the beginning of words and [ks] elsewhere. In Mexico, the letter “x” is pronounced as [h], [s] or [ks]. In words borrowed from Nahuatl, Mayan or other indigenous languages of Mexico, the letter “x” is pronounced [ʃ] or [s].
Examples: éxito, experiencia, México, Xela.
Historically, a number of Spanish consonants changed between medieval and modern Spanish.
16th Century changes to Castellano
The voiced postalveolar fricative [ʒ] (written j, or g before e and i) merged with the voiceless [ʃ] (written x, as in Quixote), and then [ʃ] evolved by the 17th century into the modern velar sound [x], now written j, or g before e and i. In much of Latin America, the same letters correspond to the [h] sound.
Further Reference:
The Pronunciation of X before Another Consonant
Richard L. Predmore
Hispania, Vol. 31, No. 2 (May, 1948) , pp. 196-197