Why is there a difference between the RAM set for KVM VPS in SolusVM and actual RAM inside VPS? For example, 1000 MB in SolusVM displayed as 990 MB of total memory in VPS.
It is expected behavior.
OS allocates a very minimal amount of RAM assigned to store the RAM mapping details.
SolusVM shows the amount of memory that will be assigned to qemu-kvm process that is responsible for running VPS. Then OS inside VPS allocates the necessary RAM for its internal usage.
Find more details about the memory allocation to a VPS on the link.