It is currently not a violation.
The Geneva Conventions apply to international armed conflicts between state parties (Ukraine and Russia are both parties). Russia has contended that it is not invading, but has actually been invited by the legitimate government in Crimea to protect Crimea from evil Ukrainian fascists. However, Article 1 of Protocol I includes a clarification that it does apply to intra-state "armed conflicts in which peoples are fighting against colonial domination and alien occupation and against racist regimes in the exercise of their right of self-determination". According to Russia's own position, combat in Crimea appears to qualify under this provision.
Wearing of uniforms
It is a widespread assumption that soldiers engaged in combat have to wear uniforms. However, looking at the text of the protocol, I only see that this is a requirement for combatants to qualify for protected POW status, and it isn't explicitly banned. Article 4 of the Third Geneva Convention says
Prisoners of war [...] are persons belonging to one of the following categories, who have fallen into the power of the enemy: [...]
- Members of other militias and members of other volunteer corps, [provided they] fulfil the following conditions:
- that of being commanded by a person responsible for his subordinates;
- that of having a fixed distinctive sign recognizable at a distance;
- that of carrying arms openly;
- that of conducting their operations in accordance with the laws and customs of war.
Members of regular armed forces qualify under the other categories, regardless of whether they're wearing a uniform. I'm not sure whether the Russian troops would be considered paramilitary or whether they're part of the regular army.
My best guess is that the Geneva Conventions are merely referring to a preexisting requirement established in customary law, as evidenced by the article several times saying "provided they carry arms openly and respect the laws and customs of war". The conclusion is that fighting without wearing a uniform might still be illegal.
There is apparently some dispute over how restrictive international law is on the uniform requirement, but regardless, all the interpretations permit Russia's actions. An article by Toni Pfanner for the Red Cross (PDF, page 12/104) vaguely refers to a belief that customary international law requires combatants to wear a uniform in hostilities (I presume the argument is that not wearing a uniform implies that you are a non-combatant, and fighting would therefore be considered perfidy). However, the author argues that the purpose of uniforms in modern international law is not to distinguish different armies, but only to distinguish combatants from civilians, as Protocol I, Article 48 says:
In order to ensure respect for and protection of the civilian population and civilian objects, the Parties to the conflict shall at all times distinguish between the civilian population and combatants and between civilian objects and military objectives and accordingly shall direct their operations only against military objectives.
Pfanner therefore concludes that
While there is a practice to wear uniforms in armies, there is not an obligation in international humanitarian law to wear them. The wearing
of civilian clothes is only illegal if it involves perfidy.
As applies to this situation, Russian soldiers are not in violation, because they are wearing military uniforms. There is no requirement that their uniforms be marked with Russian military insignia.
Furthermore, it is generally accepted that you're even allowed to wear the enemy's uniform, as long as you discard it before entering combat. (Otto Skorzeny was acquitted based on the argument that this is a legitimate ruse de guerre.) Even under the interpretation that uniforms are required in combat, there is no violation (currently), as Russia invaded unopposed (no shots fired, and no combat, thus far).