"I've eaten, shaved and showered"?

Which one is better?

1. I've eaten, shaved and showered.
2. I've eaten, shaven and showered.

The first one sounds more correct to me, but shouldn't we use the past participle shaven for the same reason that we use the past participle eaten? Why would shave take the ending -ed when eat takes the ending -en in the past participle?

Solution 1:

I would say either is possible, but I prefer shaved because it is more common. The following passage describes both shaved and shaven as possible past-participle forms in North American English:

For some verbs, neither form is particularly stigmatized for North American English speakers. Consider these examples of verbs with two past-participle options: I have sewn or I have sewed, I have shaven or I have shaved, I have mown or I have mowed, I have proven or I have proved.

(Linguistics for Everyone: An Introduction, by Kristin Denham and Anne Lobeck, p. 169)

But note that for the past-tense form, "shaved" is the only option.

the history behind the use of shaven and shaved

As David's answer says, shave comes from a "strong verb", which means that it originally lacked -ed in both the past tense and past participle forms. (The suffix -ed characterizes the verb class called "weak verbs". The terminology "strong" and "weak" refers to the way the verb inflects.) But the strong past tense of shave was lost, and the strong participle form seems to be fairly far into the process of being lost. It has been common for strong verbs to develop into weak verbs in English, although there are occasional examples of change happening in the opposite direction, with weak verbs gaining strong -⁠(e)n participles. A shift from weak to strong inflection seems to be the origin of the past-participle forms hidden, proven, sawn, shown, worn ("English Irregular Verbs", Wikipedia).

shaved is currently more common as a past participle than shaven

Although "shaven" is not entirely obsolete as a past participle, it seems to be much less common than "shaved" in this context:

(Google Ngram Viewer)

One possible weakness of this chart is that the sequence "have/has shaven" does occur in some contexts where "shaven" isn't a participle. I subtracted "have shaven heads", which seems to be the most common example, but there might be others that I didn't get.

Nevertheless, I think that the "have shaven" and "has shaven" lines in the chart above contain at least some real examples of shaven being used as a participle. Here are a few specific examples taken from Google Books of shaven being used as a past participle, just to establish that this usage really does exist for some present-day English speakers:

• [...] his bald little head that I am sure the barber across from us could have shaven stars and planets into.

  (If You Find This Letter: My Journey to Find Purpose Through Hundreds of Letters to Strangers, by Hannah Brencher, 2015, p. 155)

• It looks like her head has been freshly shaven. Maybe, she was a cancer patient and undergoing chemotherapy. May have shaven her head, because her hair was falling out.

  (Under the Baja Sun, by Shy Wright, 2012, p. 226)

people who don't use shaven as a participle may still use it as an adjective

The form "shaven" is frequently used as an adjective, even by people who don't use it as the past participle of "shave". Compare "molten", which is still used as an adjective even though it has been completely replaced by "melted" as the past-participle form of the verb "melt".

Solution 2:

Which question should one answer?

There are two questions here. The first is “which one [sentence] is better?”. That is a question of usage, which @sumelic demonstrated can be answered by reference to sources such as Google Books. The second I think is more fundamental, and is the one I shall consider:

“Why would shave take the ending -ed when eat takes the ending -en in the past participle?”

An answer

Because in English there are strong verbs and weak verbs, which form their past tenses according to different systems. Eat is an example of a strong verb, shower is an example of a weak verb, and shave is a verb that was originally strong but has changed so that it is mainly weak, but with some strong forms surviving as less-used alternatives.

This is explained in the Wikipedia article on Germanic strong verbs:

“In the Germanic languages, a strong verb is a verb that marks its past tense by means of changes to the stem vowel (ablaut). The majority of the remaining verbs form the past tense by means of a dental suffix (e.g. -ed in English), and are known as weak verbs.”

In the example sentences we are dealing with past participles — the past participles of weak verbs in English (a Germanic language) end in -ed (or -d or -t), whereas those of strong verbs usually end in -en (or -n). However to clarify let us look at three forms of the verb: the infinitive, the simple past tense (aka preterite) and the past participle used with an auxiliary (have (’ve) in the examples). In addition I have included the participial adjective (where it occurs), which, although originally formed from the past participle, does not necessarily evolve/degenerate in concert with it in the case of strong verbs.

The table shows the standard strong form with eat (eat, ate, eaten) and the weak form with shower (shower, showered, showered), together with an additional example where there is a good example of a participial adjective, drive for a strong verb (p. adj. ‘driven’, as in “the driven snow”) and escape for a weak verb (p. adj. ‘escaped’, as in “an escaped prisoner”). In the case of the verb shave, which was originally strong, I would suggest that the strong form of the past participle (shaven) is more common today as a participial adjective. A second example, shear, is provided as both strong (shore, shorn, shorn) and weak (sheared, sheared, sheared) forms can still be found in both past tenses and the participial adjective).

As a child in an English-speaking country one encounters these older strong forms in traditional texts such as nursery rhymes, even if they have fallen out of contemporary use. The example brings to mind The House that Jack Built:

This is the priest all shaven and shorn,

That married the man all tattered and torn…

Coda: Early examples of use of past participle, shaven

As there has been some discussion about the use of shaven, I checked in the OED for early examples of its use. The entry for shave is extensive, but under the meaning 3 as a verb, “to cut off… close to the skin with or as with a razor”, there is the following:

1530 He hath shavyn away all the heare on his heed

There is a separate entry for shaven as an adjective, with the following early example of use in recognizable form:

1500–1520 Quhill preistis come in with hair shevin nekkis

(Don’t ask me about ‘Quhill’.)

Footnote: Mixed verbs

Not all mixed verbs were originally strong. Some have acquired ‘strong’ forms by analogy, as in the case of the weak verb, dive, with a simple past tense in the US of dove, but in Britain dived. (Cf. drive/drove) See the Wikipedia entry on English Irregular Verbs.