- PHB p183 Darkvision:
Within a specified range, a creature with darkvision can see in darkness as if the darkness were dim light, so areas of darkness are only lightly obscured as far as that creature is concerned. However, the creature can’t discern color in darkness, only shades of gray.
- PHB p.20 racially specific (e.g. dwarf) Darkvision:
You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.
- Monster Manual p9 "monster" specific Darkvision:
A monster with darkvision can see in the dark within a specific radius. The monster can see in dim light within the radius as if it were bright light, and in darkness as if it were dim light. The monster can't discern color in darkness , only shades of gray.
RAW, using the rule of specific beats general:
- if you have darkvision without any further description it follows rule 1. (i.e. you don't get the dim light is the same as bright light benefit). This includes the spell Darkvision PHB p.230
- if you are a dwarf or elf or gnome etc. with darkvision it follows rule 2. as it is more specific than rule 1.
- if you are a "monster" it follows rule 3 as it is more specific than rule 1. This is functionally the same as rule 2. The term "monster" used here is poor as it is not a well defined in-game term.
While I know of no errata or sage advice regarding this, common sense and the way darkvision is referred to throughout the rules strongly suggests to me that darkvision is intended to be consistent and it is an oversight that rule 1 does not contain the description about dim light.
As an example of what I mean by "the way the rules are written" is this case: a Githyanki, a "monster" from the monster manual, has a darkvision spell cast on it. RAW it means it gets better darkvision than a human character with exactly the same spell: specific beats general and the Githyanki is a "monster". Clearly an unintended consequence and completely solved by darkvision being made consistent.
It is therefore a house rule in our games that darkvision follows rule 2 regardless of the source, unless specifically stated otherwise.