I've learnt that the v sound in Spanish is pronounced as a "b", however why are some words such as "por favor" and "Revolucion" pronounced as a "v"? Also, is this type of pronunciation Spanish (spoken in Spain)?
According to the Diccionario panhispánico de dudas of the Real Academia Española de la Lengua,
There's no difference in the pronunciation of b and v in Spanish: both represent nowadays the bilabial voiced sound /b/. Spanish Orthography has mantained both letters, which represented different sounds in Latin, for reasons of tradition [...] The pronunciation of v as labio-dental has never existed in Spanish, and only happens occasionally in Valencian and Majorcan speakers, and in some from Southern Catalonia, when they speak Spanish, because of the influence of their regional languages, and in some places of America by influence of Amerindian languages. In every other case, it's a mistake some people make because of an excess of scrupulousness, based in recommendations from the past, for even if the Academia had recognised in its Diccionario de Autoridades (1726-1739) that "We Spaniards don't differentiate between the pronunciation of both letters", several editions of the Academia's Ortografía and Gramática in the 18th, 19th and 20th centuries described, and even recomended, the pronunciation of v as labio-dental, believing it convenient at the time to differentiate it from b as many of the great European languages, including French and English, that were then so influential, did ; but since the 1911 Gramática, the Academia ceased to explicitly recommend this differentiation. In short, the correct pronunciation of the letter v in Spanish is identical to that of b, so there's no oral difference in our language between words such as baca and vaca, bello and vello, acerbo y acervo.
I recognise I just learnt this, as I usually differentiate both sounds when speaking.
I believe the question is why do the sounds /V/ (sonoro, bilabial, fricativo, as "Victor") and /B/ (sonoro, bilabial, oclusivo, as "Bumblebee") get used, when their written representations ('V' and 'B') are supposed to be interchangeable.
The /V/ sound is used between two vowel sounds (not the spelling, the actual phonetic sounds), while the /B/ sound is used after a consonant sound, to start an utterance (or after a pronounced pause), or when being emphasized for some reason.
que le vaya bien,
lávame (and seen on dirty windows in Chihuahua:
Vamos al cine,
In many Spanish speakers it is possible to find the differentiation between 'V' and 'B' very well marked, regardless of their country of origin, although there are some countries in which the inconsistency you refer to is more frecuently found. It is very beautiful and makes a lot of sense when you can hear these subtleties in the spoken language. I have identified it is easier to understand when they are present. In my experience, regardless of what books may say, I found it is pretty much the same in every Spanish speaking country I have been to, which suggests the ability to execute these sounds is highly linked to the speaker or a group of people, even regions or social groups, and their foundations at speaking and understanding the Spanish language, more than being linked to a specific country. This is why you will always find the inconsistencies you refer to in your question. It is part of our human nature.
Not because a definition in a dictionary renders a sound a one-way universal axiom, it means such is true and actually happens for the whole of the spoken Spanish. A mere definition can't wipe out reality. It has been said previously in this forum (source here) and in other online sources, that they learnt to make a distinction when pronouncing b and v, and that is very much alive. The fact is that there are many groups of people in Spain and America speaking with a labio-dental V and they can't be labeled as not speaking correctly because somewhere it says that THE rule is to make no distinction between both sounds (B and V). There are historical and sociological reasons for pronouncing the V labio-dental (sort of vibration if that is easier to understand. Listen here) and it even comes sometimes as a natural sound which endures in time and pervades social groups. The later has been indeed documented in the following study made in Chile:
PRODUCCIÓN DE /v/ COMO ALÓFONO DE /b/ EN NIÑOS PRE ALFABETIZADOS DE LA PROVINCIA DE CONCEPCIÓN (CHILE) by Vergara Fernández, Viviana, Boletín de Lingüística, vol. XXIII, núm. 35-36, enero-diciembre, 2011, pp. 123-142 Universidad Central de Venezuela Caracas, Venezuela.
Also we have an exploration into the existence of the labio-dental sound in certain regions in Chile:
THE VOICED LABIODENTAL ALLOPHONE [v] OF THE PHONEME /b/ IN THE SPANISH OF CONCEPCIÓN (CHILE): AN EXPLORATORY STUDY by Scott Sadowsky, Estudios de Fonética Experimental, ISSN 1575-5533, XIX, 2010, pp. 231-261 - Laboratorio de Fonética. Universidad de Concepción, Chile.
The following is an exciting and very well documented research showing why in the Spanish dialect in Chile, the sounding labio-dental fricative [v] is exercised, which entertains, in turn, the exercise in differentiating between B and V in parts of Chile. In page #10 it gives references of this differentiation being taught formally in certain educational backgrounds:
Incidencia de la alfabetización en la producción de los alófonos de /b/ by Viviana Vergara Fernández, Universidad de Concepción, Chile - 2010.
This singularity in the pronunciation of the Spanish language in Chile (although the labiodental V is to be found in other regions of America) has been registered to come also as a natural event unaffected by the educational process:
"The voiced labiodental fricative [v] has traditionally been rejected as a natural allophone of /b/ in Spanish in general, as well as in Chilean Spanish, although its existence has been admitted in cases of extra-linguistic influence, and recent research has included it in the phonetic inventory. This paper seeks to contribute by providing new data on the production of allophones of /b/, including [v]. To achieve this purpose, audio and video recordings of the speech of 19 pre-literate and literate native speakers of Spanish from Concepción, Chile, were analyzed. The results show that for all speakers, [v] is not just a natural allophone of /b/, it is the most common one". Furthermore, it is demonstrated that the existence of [v] is not affected by the literacy process. (Abstract, A study of the careful speech of pre-literate and literate speakers, by Viviana Vergara Fernández: Laboratorio de Fonética, Facultad de Humanidades y Arte, Universidad de Concepción
Although sometimes you try to teach children that there are difference in pronunciation, is that just a sign of exaggeration and sometimes pedantic oratory.
As in practice the words baca and vaca are pronounced exactly the same, spelling difference is due just to historical reasons, and their proper orthographic use is exclusively memoristic. So there is the confusion.